feat(homecore-ui iter 1): Modal + EntityForm + Add Entity flow

First CRUD increment. Click "+ Add entity" on the Dashboard
toolbar → modal opens → form with entity_id / state / attributes
fields → Create validates client-side then POSTs /api/states/<id>
→ modal closes, toast confirms, dashboard refreshes.

New components:
  frontend/src/components/Modal.ts (~110 LOC) — reusable accessible
    overlay. open property; closes on Escape and backdrop click.
    Heading prop; default + footer slots.

  frontend/src/components/EntityForm.ts (~130 LOC) — three-field form
    with public requestSubmit()/requestCancel() methods. Client-side
    validation:
      - entity_id matches /^[a-z][a-z0-9_]*\.[a-z][a-z0-9_]*$/
      - state non-empty
      - attributes parses as a JSON object (rejects array/scalar)
    Emits hc-entity-submit / hc-entity-cancel events for host to
    handle. Footer buttons live in the host (modal slot=footer).

  frontend/src/pages/Dashboard.ts (+60 LOC) — toolbar with
    "+ Add entity" button, modal state, POST handler that wraps
    fetch with bearer token, success toast (3 s), refresh().

Browser-verified end-to-end (real homecore-server :8123):
  - Toolbar button visible: Y
  - Modal opens: Y
  - 3/3 validation paths fire correctly:
      BadID → "entity_id must match domain.snake_case"
      blank state → "state must not be empty"
      [1,2,3] attrs → "attributes must be a JSON object"
  - Successful create: light.test_bulb POSTed; modal closes; toast
    "Created light.test_bulb = on"; grid count went 10 → 11
  - Persistence: hard reload, count stays
  - 0 console errors (Lit dev-mode notices excluded)

Note: TypeScript caught a name collision — `attributes` is reserved
on HTMLElement (NamedNodeMap). Renamed the Lit @property to
`entityAttrs` so the class extends LitElement cleanly.

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

frontend/src/components/EntityForm.ts

@@ -0,0 +1,143 @@
+/**
+ * `<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_]*$/
+ *   - state is non-empty
+ *   - attributes parses as a JSON object (not array, not scalar)
+ */
+
+import { LitElement, html, css } from 'lit';
+import { customElement, property, state } from 'lit/decorators.js';
+
+const ENTITY_ID_RE = /^[a-z][a-z0-9_]*\.[a-z][a-z0-9_]*$/;
+
+@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;
+
+    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; }
+        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);
+        }
+    }
+
+    /** 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=${this.entityId}
+                       ?disabled=${this.editing}
+                       @input=${(e: Event) => (this.entityId = (e.target as HTMLInputElement).value)}
+                       placeholder="light.kitchen_ceiling" />
+                <div class="hint">format: <code>domain.snake_case</code> — domain like sensor / light / switch / binary_sensor</div>
+
+                <label for="state">state</label>
+                <input id="state" .value=${this.state}
+                       @input=${(e: Event) => (this.state = (e.target as HTMLInputElement).value)}
+                       placeholder="on / off / 42 / 14.5 / detected" />
+
+                <label for="attrs">attributes (JSON object)</label>
+                <textarea id="attrs" .value=${this._attrs}
+                          @input=${(e: Event) => (this._attrs = (e.target as HTMLTextAreaElement).value)}
+                          placeholder='{ "friendly_name": "Kitchen Ceiling", "brightness": 230 }'></textarea>
+                <div class="hint">optional; leave blank for <code>{}</code></div>
+
+                ${this._err ? html`<div class="err">${this._err}</div>` : ''}
+            </form>
+        `;
+    }
+}
+
+declare global { interface HTMLElementTagNameMap { 'hc-entity-form': EntityForm; } }