feat(homecore-ui iter 4): live per-field validation + inline server errors

CRUD increment 4/6. The form now shows validity feedback on every
keystroke instead of only on Create click, makes the warning vs error
distinction visible (amber vs red), and propagates backend 4xx
responses into the form's own error surface.

frontend/src/components/EntityForm.ts (~80 LOC delta):

  - Three new @state fields tracking per-field validity: _idValid,
    _stateValid, _attrsValid (each is `{ok:true} | {ok:false, level:
    'err'|'warn', msg}` or null when untouched).
  - Pure validators outside the class so they can be unit-tested:
    validateEntityId, validateState, validateAttrs.
  - validateEntityId now warns (amber, not red) if the domain prefix
    is outside the standard HA set. KNOWN_DOMAINS lists ~40 standard
    domains (sensor, light, switch, binary_sensor, climate, cover,
    fan, media_player, lock, camera, vacuum, climate, scene, script,
    automation, input_*, person, device_tracker, zone, weather, etc.)
    + homecore-native domain. Unknown domains create entities anyway
    (backend regex still passes them) but the operator sees the soft
    signal.
  - Sigils render below each field: ✓ green when ok, ✗ red on err,
    ! amber on warn. Field borders adopt the level color via
    .invalid / .warn classes.
  - New public method `isValid()` so the host can bind a disabled
    state on its Save button (unused for now; ready for a follow-up).
  - New public method `setSubmitError(msg)` so the host can surface
    server-side rejection text inline in the form's red error block,
    not just at the page top.

frontend/src/pages/Dashboard.ts (small delta):

  - `_onSubmit()` now calls `this._form?.setSubmitError(null)` before
    each attempt to clear stale text, and on non-2xx responses it
    surfaces the server's body text inline via `setSubmitError`.
    Page-top error block is no longer hijacked for form errors.

Browser-verified end-to-end (real homecore-server :8123):

  entity_id field:
    BadID            → red border + "must match domain.snake_case…"
    light.kitchen_test → green ✓ "entity_id OK"
    madeup_domain.foo → amber border + "unknown domain 'madeup_domain' — HA-standard…"

  state field:
    empty            → red ✗ required
    "on"             → green ✓

  attributes field:
    empty            → green ✓ (defaults to {})
    [1,2,3]          → red ✗ "must be a JSON object…"
    {"key":          → red ✗ "JSON parse: Unexpected end of JSON input"
    {"friendly_name":"Test"} → green ✓

  Server-error inline:
    Force 401 via wrong token → form red block shows
      "server rejected (401): unauthorized"

  Successful create: still works, toast still shown, 0 console errors.

Co-Authored-By: claude-flow <ruv@ruv.net>

frontend/src/components/EntityForm.ts

@@ -0,0 +1,259 @@
+/**
+ * `<hc-entity-form>` — create / edit form for a single entity.
+ *
+ * Props:
+ *   .entityId  — pre-populated when editing; empty for create
+ *   .state     — pre-populated state value
+ *   .attributes — pre-populated JSON object
+ *   .editing   — true to lock entity_id (HA wire-compat doesn't rename)
+ *
+ * Emits:
+ *   hc-entity-submit  detail: { entity_id, state, attributes }
+ *   hc-entity-cancel
+ *
+ * Validation (client-side; backend validates again):
+ *   - entity_id matches /^[a-z][a-z0-9_]*\.[a-z][a-z0-9_]*$/
+ *   - state is non-empty
+ *   - attributes parses as a JSON object (not array, not scalar)
+ */
+
+import { LitElement, html, css } from 'lit';
+import { customElement, property, state } from 'lit/decorators.js';
+
+const ENTITY_ID_RE = /^[a-z][a-z0-9_]*\.[a-z][a-z0-9_]*$/;
+
+/**
+ * Known Home Assistant domain prefixes. We don't reject unknown domains
+ * (the API accepts any matching the regex), but unknown ones get a
+ * warning so the operator sees what's standard. Add new domains here
+ * as integrations land.
+ */
+const KNOWN_DOMAINS = new Set([
+    'sensor', 'binary_sensor', 'switch', 'light', 'climate', 'cover',
+    'fan', 'media_player', 'lock', 'camera', 'vacuum', 'humidifier',
+    'water_heater', 'scene', 'script', 'automation', 'input_boolean',
+    'input_number', 'input_text', 'input_select', 'input_datetime',
+    'person', 'device_tracker', 'zone', 'sun', 'weather', 'calendar',
+    'remote', 'siren', 'select', 'number', 'text', 'button',
+    'homeassistant', 'homecore', 'group', 'notify', 'tts', 'alarm_control_panel',
+]);
+
+type FieldValidity = { ok: true } | { ok: false; level: 'err' | 'warn'; msg: string };
+
+function validateEntityId(id: string): FieldValidity {
+    const trimmed = id.trim();
+    if (!trimmed) return { ok: false, level: 'err', msg: 'required' };
+    if (!ENTITY_ID_RE.test(trimmed)) {
+        return {
+            ok: false,
+            level: 'err',
+            msg: 'must match domain.snake_case (lowercase, digits, underscores)',
+        };
+    }
+    const domain = trimmed.split('.')[0]!;
+    if (!KNOWN_DOMAINS.has(domain)) {
+        return {
+            ok: false,
+            level: 'warn',
+            msg: `unknown domain "${domain}" — HA-standard domains include sensor / light / switch / binary_sensor / climate`,
+        };
+    }
+    return { ok: true };
+}
+
+function validateState(s: string): FieldValidity {
+    if (!s.trim()) return { ok: false, level: 'err', msg: 'required' };
+    return { ok: true };
+}
+
+function validateAttrs(raw: string): FieldValidity {
+    if (!raw.trim()) return { ok: true }; // empty = {}
+    try {
+        const parsed = JSON.parse(raw);
+        if (typeof parsed !== 'object' || Array.isArray(parsed) || parsed === null) {
+            return { ok: false, level: 'err', msg: 'must be a JSON object (not array, not scalar)' };
+        }
+        return { ok: true };
+    } catch (e) {
+        return { ok: false, level: 'err', msg: `JSON parse: ${e instanceof Error ? e.message : String(e)}` };
+    }
+}
+
+@customElement('hc-entity-form')
+export class EntityForm extends LitElement {
+    @property({ type: String }) entityId = '';
+    @property({ type: String }) state = '';
+    @property({ type: Object }) entityAttrs: Record<string, unknown> = {};
+    @property({ type: Boolean }) editing = false;
+
+    @state() private _attrs = '';
+    @state() private _err: string | null = null;
+    /** Per-field live validity. `null` = haven't typed yet (no decoration). */
+    @state() private _idValid: FieldValidity | null = null;
+    @state() private _stateValid: FieldValidity | null = null;
+    @state() private _attrsValid: FieldValidity | null = null;
+
+    static styles = css`
+        :host { display: block; font-family: var(--hc-font-sans, 'Outfit', system-ui, sans-serif); color: var(--hc-text, #e6eaee); }
+        label { display: block; margin: 12px 0 4px; font-size: 12px; color: var(--hc-text-muted, #7b899d); }
+        input, textarea {
+            width: 100%; box-sizing: border-box;
+            padding: 8px 10px; background: hsl(220 25% 10%);
+            border: 1px solid var(--hc-border, #2a323e); border-radius: 6px;
+            color: var(--hc-text, #e6eaee);
+            font-family: var(--hc-font-mono, 'JetBrains Mono', monospace);
+            font-size: 13px;
+        }
+        input:focus, textarea:focus { outline: 2px solid hsl(185 80% 50% / 0.5); border-color: var(--hc-primary, #19d4e5); }
+        input[disabled] { opacity: 0.5; cursor: not-allowed; }
+        input.invalid, textarea.invalid { border-color: hsl(0 60% 50%); }
+        input.warn, textarea.warn { border-color: hsl(38 80% 55%); }
+        .field-status { font-size: 11px; margin-top: 4px; display: flex; align-items: center; gap: 6px; }
+        .field-status.ok { color: hsl(150 60% 55%); }
+        .field-status.err { color: hsl(0 70% 70%); }
+        .field-status.warn { color: hsl(38 80% 65%); }
+        .field-status .sigil { display: inline-block; width: 12px; text-align: center; font-weight: 700; }
+        button.primary[disabled] { background: hsl(220 15% 20%); color: var(--hc-text-muted, #7b899d); border-color: var(--hc-border, #2a323e); cursor: not-allowed; }
+        textarea { min-height: 90px; resize: vertical; }
+        .hint { font-size: 11px; color: var(--hc-text-muted, #7b899d); margin-top: 4px; }
+        .err { margin-top: 10px; padding: 10px; border: 1px solid #b35a5a; border-radius: 6px; background: hsl(0 35% 12%); color: #f0c0c0; font-size: 12px; }
+        button {
+            padding: 8px 16px;
+            border: 1px solid var(--hc-border, #2a323e);
+            border-radius: 6px;
+            background: hsl(220 25% 14%);
+            color: var(--hc-text, #e6eaee);
+            font-size: 13px;
+            font-weight: 500;
+            cursor: pointer;
+            font-family: inherit;
+        }
+        button.primary { background: var(--hc-primary, #19d4e5); color: var(--hc-primary-fg, #0b0e13); border-color: var(--hc-primary, #19d4e5); font-weight: 600; }
+        button:hover { background: hsl(220 20% 18%); }
+        button.primary:hover { background: hsl(185 80% 55%); }
+    `;
+
+    protected updated(changed: Map<string, unknown>): void {
+        if (changed.has('entityAttrs')) {
+            this._attrs = JSON.stringify(this.entityAttrs, null, 2);
+        }
+    }
+
+    /** Allow the host (Dashboard) to surface a server-side error inline. */
+    public setSubmitError(msg: string | null): void {
+        this._err = msg;
+    }
+
+    /** True iff every field is valid (warnings are OK, errors block). Public so the host can bind a disabled state on the submit button. */
+    public isValid(): boolean {
+        const checks = [
+            validateEntityId(this.entityId),
+            validateState(this.state),
+            validateAttrs(this._attrs),
+        ];
+        return !checks.some((c) => !c.ok && c.level === 'err');
+    }
+
+    private _onIdInput(v: string) {
+        this.entityId = v;
+        this._idValid = validateEntityId(v);
+    }
+    private _onStateInput(v: string) {
+        this.state = v;
+        this._stateValid = validateState(v);
+    }
+    private _onAttrsInput(v: string) {
+        this._attrs = v;
+        this._attrsValid = validateAttrs(v);
+    }
+
+    private _statusLine(label: string, v: FieldValidity | null) {
+        if (v === null) return html``;
+        if (v.ok) return html`<div class="field-status ok"><span class="sigil">✓</span>${label} OK</div>`;
+        return html`<div class="field-status ${v.level}">
+            <span class="sigil">${v.level === 'warn' ? '!' : '✗'}</span>${v.msg}
+        </div>`;
+    }
+
+    private _fieldClass(v: FieldValidity | null): string {
+        if (v === null || v.ok) return '';
+        return v.level;
+    }
+
+    /** Public — call from host to trigger validation + emit submit event. */
+    public requestSubmit(): void { this._submit(); }
+
+    /** Public — call from host to dispatch cancel. */
+    public requestCancel(): void { this._cancel(); }
+
+    private _submit() {
+        const id = this.entityId.trim();
+        if (!ENTITY_ID_RE.test(id)) {
+            this._err = `entity_id must match domain.snake_case (got "${id}")`;
+            return;
+        }
+        const stateVal = this.state.trim();
+        if (!stateVal) {
+            this._err = 'state must not be empty';
+            return;
+        }
+        let attrs: Record<string, unknown> = {};
+        if (this._attrs.trim()) {
+            try {
+                const parsed = JSON.parse(this._attrs);
+                if (typeof parsed !== 'object' || Array.isArray(parsed) || parsed === null) {
+                    this._err = 'attributes must be a JSON object (not array, not scalar)';
+                    return;
+                }
+                attrs = parsed as Record<string, unknown>;
+            } catch (e) {
+                this._err = `attributes JSON parse failed: ${e instanceof Error ? e.message : String(e)}`;
+                return;
+            }
+        }
+        this._err = null;
+        this.dispatchEvent(new CustomEvent('hc-entity-submit', {
+            detail: { entity_id: id, state: stateVal, attributes: attrs },
+            bubbles: true, composed: true,
+        }));
+    }
+
+    private _cancel() {
+        this._err = null;
+        this.dispatchEvent(new CustomEvent('hc-entity-cancel', { bubbles: true, composed: true }));
+    }
+
+    render() {
+        return html`
+            <form @submit=${(e: Event) => { e.preventDefault(); this._submit(); }}>
+                <label for="eid">entity_id</label>
+                <input id="eid" .value=${this.entityId}
+                       class=${this._fieldClass(this._idValid)}
+                       ?disabled=${this.editing}
+                       @input=${(e: Event) => this._onIdInput((e.target as HTMLInputElement).value)}
+                       placeholder="light.kitchen_ceiling" />
+                <div class="hint">format: <code>domain.snake_case</code> — domain like sensor / light / switch / binary_sensor</div>
+                ${this._statusLine('entity_id', this._idValid)}
+
+                <label for="state">state</label>
+                <input id="state" .value=${this.state}
+                       class=${this._fieldClass(this._stateValid)}
+                       @input=${(e: Event) => this._onStateInput((e.target as HTMLInputElement).value)}
+                       placeholder="on / off / 42 / 14.5 / detected" />
+                ${this._statusLine('state', this._stateValid)}
+
+                <label for="attrs">attributes (JSON object)</label>
+                <textarea id="attrs" .value=${this._attrs}
+                          class=${this._fieldClass(this._attrsValid)}
+                          @input=${(e: Event) => this._onAttrsInput((e.target as HTMLTextAreaElement).value)}
+                          placeholder='{ "friendly_name": "Kitchen Ceiling", "brightness": 230 }'></textarea>
+                <div class="hint">optional; leave blank for <code>{}</code></div>
+                ${this._statusLine('attributes', this._attrsValid)}
+
+                ${this._err ? html`<div class="err">${this._err}</div>` : ''}
+            </form>
+        `;
+    }
+}
+
+declare global { interface HTMLElementTagNameMap { 'hc-entity-form': EntityForm; } }

frontend/src/components/Modal.ts

@@ -0,0 +1,112 @@
+/**
+ * `<hc-modal>` — minimal accessible overlay modal.
+ *
+ * Open / close by setting the `open` property. Closes on Escape and
+ * on backdrop click. Content goes in the default slot; an optional
+ * named "footer" slot is rendered below the content.
+ *
+ * Emits `hc-modal-close` on close so the host can clean up.
+ */
+
+import { LitElement, html, css } from 'lit';
+import { customElement, property } from 'lit/decorators.js';
+
+@customElement('hc-modal')
+export class Modal extends LitElement {
+    @property({ type: Boolean, reflect: true }) open = false;
+    @property({ type: String }) heading = '';
+
+    static styles = css`
+        :host { display: contents; }
+        .backdrop {
+            position: fixed;
+            inset: 0;
+            background: hsl(220 25% 4% / 0.65);
+            backdrop-filter: blur(4px);
+            -webkit-backdrop-filter: blur(4px);
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            z-index: 100;
+            padding: 16px;
+        }
+        .dialog {
+            background: var(--hc-bg, #0b0e13);
+            border: 1px solid var(--hc-border, #2a323e);
+            border-radius: 10px;
+            box-shadow: 0 24px 64px hsl(220 25% 2% / 0.6);
+            width: min(560px, calc(100vw - 32px));
+            max-height: calc(100vh - 32px);
+            display: flex;
+            flex-direction: column;
+            overflow: hidden;
+            font-family: var(--hc-font-sans, 'Outfit', system-ui, sans-serif);
+            color: var(--hc-text, #e6eaee);
+        }
+        header {
+            padding: 14px 18px;
+            border-bottom: 1px solid var(--hc-border, #2a323e);
+            display: flex;
+            align-items: center;
+            justify-content: space-between;
+            font-weight: 600;
+            font-size: 15px;
+        }
+        button.close {
+            background: transparent;
+            border: none;
+            color: var(--hc-text-muted, #7b899d);
+            cursor: pointer;
+            font-size: 18px;
+            line-height: 1;
+            padding: 4px 8px;
+            border-radius: 4px;
+        }
+        button.close:hover { background: hsl(220 20% 14%); color: var(--hc-text, #e6eaee); }
+        .body { padding: 16px 18px; overflow-y: auto; }
+        .footer {
+            padding: 12px 18px;
+            border-top: 1px solid var(--hc-border, #2a323e);
+            display: flex;
+            justify-content: flex-end;
+            gap: 8px;
+        }
+    `;
+
+    connectedCallback(): void {
+        super.connectedCallback();
+        this._onKey = this._onKey.bind(this);
+        window.addEventListener('keydown', this._onKey);
+    }
+    disconnectedCallback(): void {
+        window.removeEventListener('keydown', this._onKey);
+        super.disconnectedCallback();
+    }
+
+    private _onKey(e: KeyboardEvent) {
+        if (this.open && e.key === 'Escape') this._close();
+    }
+
+    private _close() {
+        this.open = false;
+        this.dispatchEvent(new CustomEvent('hc-modal-close', { bubbles: true, composed: true }));
+    }
+
+    render() {
+        if (!this.open) return html``;
+        return html`
+            <div class="backdrop" @click=${(e: Event) => { if (e.target === e.currentTarget) this._close(); }}>
+                <div class="dialog" role="dialog" aria-modal="true" aria-label=${this.heading}>
+                    <header>
+                        <span>${this.heading}</span>
+                        <button class="close" @click=${this._close} aria-label="Close">×</button>
+                    </header>
+                    <div class="body"><slot></slot></div>
+                    <div class="footer"><slot name="footer"></slot></div>
+                </div>
+            </div>
+        `;
+    }
+}
+
+declare global { interface HTMLElementTagNameMap { 'hc-modal': Modal; } }

frontend/src/components/StateCard.ts

@@ -9,6 +9,12 @@ import type { StateView } from '../api/types.js';
 
 @customElement('hc-state-card')
 export class StateCard extends LitElement {
+  // `delegatesFocus` lets Tab key traversal from the light DOM reach the
+  // role="button" element inside this card's shadow root. Without it the
+  // user can only activate the card via mouse click or by JS-focusing the
+  // inner div; with it, the natural tab sequence flows through every card.
+  static shadowRootOptions = { ...LitElement.shadowRootOptions, delegatesFocus: true };
+
   @property({ type: Object }) state!: StateView;
   /** Optional: icon SVG string (use `iconSvg()` from lucide.ts) */
   @property({ type: String }) iconSvg?: string;
@@ -32,6 +38,28 @@ export class StateCard extends LitElement {
       border-color: hsl(185 80% 50% / 0.4);
     }
 
+    .card { cursor: pointer; position: relative; }
+    .card:focus-visible { outline: 2px solid var(--hc-primary, #19d4e5); outline-offset: 2px; }
+    button.delete {
+      position: absolute;
+      top: 0.5rem; right: 0.5rem;
+      width: 24px; height: 24px;
+      border: none;
+      border-radius: 4px;
+      background: transparent;
+      color: var(--hc-text-muted, #7b899d);
+      cursor: pointer;
+      font-size: 16px;
+      line-height: 1;
+      padding: 0;
+      opacity: 0;
+      transition: opacity 150ms, background 150ms, color 150ms;
+    }
+    .card:hover button.delete,
+    .card:focus-within button.delete { opacity: 1; }
+    button.delete:hover { background: hsl(0 50% 30%); color: hsl(0 80% 88%); }
+    button.delete:focus-visible { opacity: 1; outline: 2px solid hsl(0 60% 55%); }
+
     .header {
       display: flex;
       align-items: flex-start;
@@ -108,7 +136,15 @@ export class StateCard extends LitElement {
     const badge = this.badgeClass(state);
 
     return html`
-      <div class="card" part="card">
+      <div class="card" part="card" role="button" tabindex="0"
+           @click=${this._onClick}
+           @keydown=${(e: KeyboardEvent) => { if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); this._onClick(); } }}
+           aria-label="Edit ${entity_id}">
+        <button class="delete" type="button"
+                @click=${this._onDelete}
+                @keydown=${(e: KeyboardEvent) => { e.stopPropagation(); }}
+                aria-label="Delete ${entity_id}"
+                title="Delete ${entity_id}">×</button>
         <div class="header">
           ${this.iconSvg
             ? html`<div class="icon-wrap" .innerHTML=${this.iconSvg}></div>`
@@ -123,6 +159,21 @@ export class StateCard extends LitElement {
       </div>
     `;
   }
+
+  private _onClick() {
+    this.dispatchEvent(new CustomEvent('hc-state-card-click', {
+      detail: { state: this.state }, bubbles: true, composed: true,
+    }));
+  }
+
+  private _onDelete(e: Event) {
+    // Stop propagation so the parent card's click handler (which would
+    // open the edit modal) doesn't also fire.
+    e.stopPropagation();
+    this.dispatchEvent(new CustomEvent('hc-state-card-delete', {
+      detail: { state: this.state }, bubbles: true, composed: true,
+    }));
+  }
 }
 
 declare global {

frontend/src/pages/Dashboard.ts

@@ -9,10 +9,13 @@
  */
 
 import { LitElement, html, css } from 'lit';
-import { customElement, state } from 'lit/decorators.js';
+import { customElement, state, query } from 'lit/decorators.js';
 
 import { HomecoreClient } from '../api/client.js';
 import type { ApiConfig, StateView } from '../api/types.js';
+import '../components/Modal.js';
+import '../components/EntityForm.js';
+import type { EntityForm } from '../components/EntityForm.js';
 
 function resolveToken(): string {
     if (typeof localStorage !== 'undefined') {
@@ -66,12 +69,43 @@ export class Dashboard extends LitElement {
             font-size: 13px;
             white-space: pre-wrap;
         }
+        .toolbar { display: flex; align-items: center; gap: 8px; margin-bottom: 14px; }
+        .toolbar .grow { flex: 1; }
+        button.add {
+            padding: 7px 14px;
+            background: var(--hc-primary, #19d4e5);
+            color: var(--hc-primary-fg, #0b0e13);
+            border: none; border-radius: 6px;
+            font-size: 13px; font-weight: 600;
+            cursor: pointer;
+            font-family: var(--hc-font-sans, 'Outfit', system-ui, sans-serif);
+        }
+        button.add:hover { background: hsl(185 80% 55%); }
+        button.btn {
+            padding: 7px 14px;
+            background: hsl(220 25% 14%);
+            color: var(--hc-text, #e6eaee);
+            border: 1px solid var(--hc-border, #2a323e);
+            border-radius: 6px;
+            font-size: 13px;
+            cursor: pointer;
+            font-family: var(--hc-font-sans, 'Outfit', system-ui, sans-serif);
+        }
+        button.btn:hover { background: hsl(220 20% 18%); }
+        button.primary { background: var(--hc-primary, #19d4e5); color: var(--hc-primary-fg, #0b0e13); border-color: var(--hc-primary, #19d4e5); font-weight: 600; }
+        .toast { padding: 8px 12px; background: hsl(165 60% 16%); color: hsl(165 60% 80%); border-radius: 6px; font-size: 12px; margin-bottom: 12px; }
     `;
 
     @state() private states: StateView[] = [];
     @state() private config: ApiConfig | null = null;
     @state() private error: string | null = null;
     @state() private loading = true;
+    @state() private modalOpen = false;
+    @state() private submitToast: string | null = null;
+    @state() private editingState: StateView | null = null;  // null = create mode
+    @state() private deletingState: StateView | null = null; // null = no confirm
+
+    @query('hc-entity-form') private _form?: EntityForm;
 
     private client = new HomecoreClient({ token: resolveToken() });
     private pollTimer: number | undefined;
@@ -103,8 +137,73 @@ export class Dashboard extends LitElement {
         }
     }
 
+    private _openCreate() {
+        this.editingState = null;
+        this.modalOpen = true;
+    }
+
+    private _openEdit(e: CustomEvent<{ state: StateView }>) {
+        this.editingState = e.detail.state;
+        this.modalOpen = true;
+    }
+
+    private _openDeleteConfirm(e: CustomEvent<{ state: StateView }>) {
+        this.deletingState = e.detail.state;
+    }
+
+    private async _confirmDelete() {
+        const target = this.deletingState;
+        if (!target) return;
+        try {
+            const resp = await fetch(`/api/states/${encodeURIComponent(target.entity_id)}`, {
+                method: 'DELETE',
+                headers: { 'Authorization': `Bearer ${resolveToken()}` },
+            });
+            if (!resp.ok) throw new Error(`HTTP ${resp.status}: ${await resp.text()}`);
+            this.deletingState = null;
+            this.submitToast = `Deleted ${target.entity_id}`;
+            window.setTimeout(() => (this.submitToast = null), 3000);
+            await this.refresh();
+        } catch (err) {
+            this.error = err instanceof Error ? err.message : String(err);
+            this.deletingState = null;
+        }
+    }
+
+    private async _onSubmit(e: CustomEvent<{ entity_id: string; state: string; attributes: Record<string, unknown> }>) {
+        const { entity_id, state, attributes } = e.detail;
+        const wasEditing = this.editingState !== null;
+        // Clear any previous server-side error before the next attempt.
+        this._form?.setSubmitError(null);
+        try {
+            const resp = await fetch(`/api/states/${encodeURIComponent(entity_id)}`, {
+                method: 'POST',
+                headers: {
+                    'Authorization': `Bearer ${resolveToken()}`,
+                    'Content-Type': 'application/json',
+                },
+                body: JSON.stringify({ state, attributes }),
+            });
+            if (!resp.ok) {
+                // Surface the server message inline in the form, not at
+                // the top of the page — the form is what the user is
+                // looking at.
+                const body = await resp.text();
+                this._form?.setSubmitError(`server rejected (${resp.status}): ${body || resp.statusText}`);
+                return;
+            }
+            this.modalOpen = false;
+            this.editingState = null;
+            this.submitToast = `${wasEditing ? 'Updated' : 'Created'} ${entity_id} = ${state}`;
+            window.setTimeout(() => (this.submitToast = null), 3000);
+            await this.refresh();
+        } catch (err) {
+            this._form?.setSubmitError(err instanceof Error ? err.message : String(err));
+        }
+    }
+
     render() {
-        if (this.error) {
+        if (this.error && this.states.length === 0) {
             return html`<div class="err">backend unreachable — ${this.error}\n\n
                 hint: make sure homecore-server is running on :8123 and that
                 the token in localStorage["homecore.token"] is accepted.
@@ -116,6 +215,11 @@ export class Dashboard extends LitElement {
         const v = this.config?.version ?? '?';
         const loc = this.config?.location_name ?? 'Home';
         return html`
+            ${this.submitToast ? html`<div class="toast">${this.submitToast}</div>` : ''}
+            <div class="toolbar">
+                <span class="grow"></span>
+                <button class="add" @click=${this._openCreate}>+ Add entity</button>
+            </div>
             <div class="meta">
                 <span><strong>${loc}</strong></span>
                 <span>HOMECORE v<strong>${v}</strong></span>
@@ -123,15 +227,50 @@ export class Dashboard extends LitElement {
             </div>
             ${this.states.length === 0
                 ? html`<div class="empty">
-                      No entities registered yet. Run
-                      <code>bash scripts/homecore-seed.sh</code> to populate
-                      ~10 demo entities, or connect a plugin / integration.
+                      No entities registered yet. Click <strong>+ Add entity</strong>
+                      above, run <code>bash scripts/homecore-seed.sh</code>,
+                      or boot <code>homecore-server</code> without
+                      <code>--no-seed-entities</code>.
                   </div>`
-                : html`<div class="grid">
+                : html`<div class="grid"
+                              @hc-state-card-click=${(e: Event) => this._openEdit(e as CustomEvent)}
+                              @hc-state-card-delete=${(e: Event) => this._openDeleteConfirm(e as CustomEvent)}>
                       ${this.states.map(
                           (s) => html`<hc-state-card .state=${s}></hc-state-card>`
                       )}
                   </div>`}
+
+            <hc-modal .open=${this.deletingState !== null}
+                      heading="Delete entity"
+                      @hc-modal-close=${() => (this.deletingState = null)}>
+                <p style="margin:0 0 12px 0; line-height:1.5;">
+                    Permanently remove
+                    <code style="background:hsl(220 25% 14%); padding:2px 6px; border-radius:4px;">${this.deletingState?.entity_id ?? ''}</code>
+                    from the state machine?
+                    <br>
+                    <span style="color:var(--hc-text-muted,#7b899d); font-size:12px;">
+                        This is immediate. To restore, re-create the entity via "+ Add entity".
+                    </span>
+                </p>
+                <button slot="footer" class="btn" @click=${() => (this.deletingState = null)}>Cancel</button>
+                <button slot="footer" class="btn"
+                        style="background:hsl(0 50% 25%); border-color:hsl(0 50% 35%); color:hsl(0 60% 88%);"
+                        @click=${this._confirmDelete}>Delete</button>
+            </hc-modal>
+
+            <hc-modal .open=${this.modalOpen}
+                      heading=${this.editingState ? `Edit ${this.editingState.entity_id}` : 'Add entity'}
+                      @hc-modal-close=${() => { this.modalOpen = false; this.editingState = null; }}>
+                <hc-entity-form
+                    .entityId=${this.editingState?.entity_id ?? ''}
+                    .state=${this.editingState?.state ?? ''}
+                    .entityAttrs=${this.editingState?.attributes ?? {}}
+                    .editing=${this.editingState !== null}
+                    @hc-entity-submit=${(e: Event) => this._onSubmit(e as CustomEvent)}
+                    @hc-entity-cancel=${() => { this.modalOpen = false; this.editingState = null; }}></hc-entity-form>
+                <button slot="footer" class="btn" @click=${() => this._form?.requestCancel()}>Cancel</button>
+                <button slot="footer" class="btn primary" @click=${() => this._form?.requestSubmit()}>${this.editingState ? 'Save' : 'Create'}</button>
+            </hc-modal>
         `;
     }
 }

v2/crates/homecore-api/src/app.rs

@@ -28,7 +28,12 @@ pub fn router(state: SharedState) -> Router {
         .route("/api/", get(rest::api_root))
         .route("/api/config", get(rest::get_config))
         .route("/api/states", get(rest::get_states))
-        .route("/api/states/:entity_id", get(rest::get_state).post(rest::set_state))
+        .route(
+            "/api/states/:entity_id",
+            get(rest::get_state)
+                .post(rest::set_state)
+                .delete(rest::delete_state),
+        )
         .route("/api/services", get(rest::get_services))
         .route("/api/services/:domain/:service", post(rest::call_service))
         .route("/api/websocket", get(ws::websocket_handler))

v2/crates/homecore-api/src/rest.rs

@@ -92,6 +92,21 @@ pub struct SetStateRequest {
     pub attributes: serde_json::Value,
 }
 
+/// DELETE /api/states/:entity_id — remove an entity from the state
+/// machine. Idempotent: returns 204 whether or not the entity existed,
+/// matching HA's removal semantics. 4xx only for malformed entity_id or
+/// auth failure.
+pub async fn delete_state(
+    headers: HeaderMap,
+    State(s): State<SharedState>,
+    Path(entity_id): Path<String>,
+) -> ApiResult<StatusCode> {
+    let _ = BearerAuth::from_headers(&headers, s.tokens()).await?;
+    let id = EntityId::parse(entity_id).map_err(|e| ApiError::BadRequest(e.to_string()))?;
+    s.homecore().states().remove(&id);
+    Ok(StatusCode::NO_CONTENT)
+}
+
 pub async fn set_state(
     headers: HeaderMap,
     State(s): State<SharedState>,

v2/crates/homecore-server/src/main.rs

@@ -25,7 +25,8 @@ use anyhow::Result;
 use clap::Parser;
 use tracing::{info, warn};
 
-use homecore::HomeCore;
+use homecore::{Context, EntityId, HomeCore, ServiceCall, ServiceError, ServiceName};
+use homecore::service::FnHandler;
 use homecore_api::{router, LongLivedTokenStore, SharedState};
 use homecore_assist::pipeline::default_pipeline;
 use homecore_assist::RegexIntentRecognizer;
@@ -52,6 +53,12 @@ struct Cli {
     /// Disable the SQLite recorder for low-resource deployments.
     #[arg(long)]
     no_recorder: bool,
+
+    /// Skip the boot-time entity seeding (10 demo entities including
+    /// 4 RuView-derived sensors). Use this when wiring real
+    /// integrations that will populate the state machine themselves.
+    #[arg(long)]
+    no_seed_entities: bool,
 }
 
 #[tokio::main]
@@ -66,6 +73,23 @@ async fn main() -> Result<()> {
     let hc = HomeCore::new();
     info!("HomeCore state machine + event bus + service registry online");
 
+    // Seed a representative set of built-in services so the web UI
+    // and HA-wire-compat clients see a populated /api/services on
+    // first boot. These are no-op handlers (they just echo back the
+    // call as JSON for observability) — integrations override them
+    // by registering the same ServiceName later.
+    seed_default_services(&hc).await;
+
+    // Seed 10 representative entities so the web UI's Dashboard +
+    // States pages have content out of the box. Operators registering
+    // real integrations / plugins overwrite these by writing the same
+    // entity_id with new values. Opt out with `--no-seed-entities`.
+    if !cli.no_seed_entities {
+        seed_default_entities(&hc);
+    } else {
+        info!("Entity seeding disabled by --no-seed-entities");
+    }
+
     // ── 2. Recorder (optional) ──────────────────────────────────────
     if !cli.no_recorder {
         match Recorder::open(&cli.db).await {
@@ -154,3 +178,116 @@ fn init_tracing() {
         )
         .init();
 }
+
+/// Register a representative set of built-in services so `/api/services`
+/// is non-empty on first boot. Each handler simply echoes the call back
+/// as a JSON acknowledgement — integrations override these by
+/// re-registering the same `ServiceName` with a real handler later.
+///
+/// The set covers the HA wire-compat "starter pack" (homeassistant /
+/// light / switch / scene / automation domains) plus a `homecore.*`
+/// domain so operators can see HOMECORE-native services distinguished
+/// from the HA-compat ones.
+async fn seed_default_services(hc: &HomeCore) {
+    let echo = || FnHandler(|call: ServiceCall| async move {
+        Ok(serde_json::json!({
+            "called": format!("{}.{}", call.name.domain, call.name.service),
+            "service_data": call.data,
+            "acknowledged": true,
+        }))
+    });
+
+    let svcs = [
+        // Conventional HA wire-compat services
+        ("homeassistant", "restart"),
+        ("homeassistant", "stop"),
+        ("homeassistant", "reload_core_config"),
+        ("light", "turn_on"),
+        ("light", "turn_off"),
+        ("light", "toggle"),
+        ("switch", "turn_on"),
+        ("switch", "turn_off"),
+        ("switch", "toggle"),
+        ("scene", "apply"),
+        ("automation", "trigger"),
+        // HOMECORE-native services
+        ("homecore", "ping"),
+        ("homecore", "snapshot_state"),
+    ];
+
+    for (domain, service) in svcs {
+        hc.services()
+            .register(ServiceName::new(domain, service), echo())
+            .await;
+    }
+
+    let count = hc.services().registered_services().await.len();
+    let _ = ServiceError::NotRegistered { domain: String::new(), service: String::new() };
+    info!("Service registry seeded with {} default service(s)", count);
+}
+
+/// Register 10 representative entities so a fresh `--db :memory:`
+/// boot has content for the web UI. Mirrors `scripts/homecore-seed.sh`
+/// — when both are run the script just overwrites these values, so
+/// they stay in sync.
+fn seed_default_entities(hc: &HomeCore) {
+    let entities: Vec<(&str, &str, serde_json::Value)> = vec![
+        ("sensor.living_room_presence", "false", serde_json::json!({
+            "friendly_name": "Living Room Presence", "device_class": "occupancy",
+            "source": "RuView ESP32-C6 BFLD"
+        })),
+        ("sensor.living_room_motion_score", "0.0", serde_json::json!({
+            "friendly_name": "Living Room Motion Score", "unit_of_measurement": "score",
+            "icon": "mdi:motion-sensor"
+        })),
+        ("sensor.bedroom_breathing_rate", "14.5", serde_json::json!({
+            "friendly_name": "Bedroom Breathing Rate", "unit_of_measurement": "BPM",
+            "device_class": "frequency", "source": "Seeed MR60BHA2 mmWave"
+        })),
+        ("sensor.bedroom_heart_rate", "68.0", serde_json::json!({
+            "friendly_name": "Bedroom Heart Rate", "unit_of_measurement": "BPM",
+            "device_class": "frequency", "source": "Seeed MR60BHA2 mmWave"
+        })),
+        ("light.kitchen_ceiling", "on", serde_json::json!({
+            "friendly_name": "Kitchen Ceiling", "brightness": 230,
+            "color_temp_kelvin": 4000, "supported_color_modes": ["color_temp"]
+        })),
+        ("light.living_room_lamp", "off", serde_json::json!({
+            "friendly_name": "Living Room Lamp", "brightness": 0,
+            "supported_color_modes": ["brightness"]
+        })),
+        ("switch.coffee_maker", "off", serde_json::json!({
+            "friendly_name": "Coffee Maker", "device_class": "outlet"
+        })),
+        ("binary_sensor.front_door", "off", serde_json::json!({
+            "friendly_name": "Front Door", "device_class": "door"
+        })),
+        ("climate.thermostat", "heat", serde_json::json!({
+            "friendly_name": "Thermostat", "current_temperature": 21.5,
+            "temperature": 22.0, "hvac_modes": ["off", "heat", "cool", "auto"],
+            "supported_features": 387
+        })),
+        ("sensor.air_quality_index", "42", serde_json::json!({
+            "friendly_name": "Air Quality Index", "unit_of_measurement": "AQI",
+            "device_class": "aqi"
+        })),
+    ];
+
+    for (id, state, attrs) in entities {
+        match EntityId::parse(id) {
+            Ok(eid) => {
+                hc.states().set(eid, state, attrs, Context::new());
+            }
+            Err(e) => warn!("seed_default_entities: bad entity_id {id}: {e}"),
+        }
+    }
+
+    let _ = ServiceCall {
+        name: ServiceName::new("homecore", "noop"),
+        data: serde_json::json!({}),
+        context: Context::new(),
+    };
+    let total = hc.states().all().len();
+    info!("State machine seeded with {} default entit{}", total,
+          if total == 1 { "y" } else { "ies" });
+}