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/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 +69,41 @@ 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;
+
+    @query('hc-entity-form') private _form?: EntityForm;
 
     private client = new HomecoreClient({ token: resolveToken() });
     private pollTimer: number | undefined;
@@ -103,8 +135,30 @@ export class Dashboard extends LitElement {
         }
     }
 
+    private async _onSubmit(e: CustomEvent<{ entity_id: string; state: string; attributes: Record<string, unknown> }>) {
+        const { entity_id, state, attributes } = e.detail;
+        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) throw new Error(`HTTP ${resp.status}: ${await resp.text()}`);
+            this.modalOpen = false;
+            this.submitToast = `Created ${entity_id} = ${state}`;
+            window.setTimeout(() => (this.submitToast = null), 3000);
+            await this.refresh();
+        } catch (err) {
+            // Form-level error stays in the form; surface at top too for visibility.
+            this.error = 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 +170,11 @@ 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=${() => (this.modalOpen = true)}>+ Add entity</button>
+            </div>
             <div class="meta">
                 <span><strong>${loc}</strong></span>
                 <span>HOMECORE v<strong>${v}</strong></span>
@@ -123,15 +182,25 @@ 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">
                       ${this.states.map(
                           (s) => html`<hc-state-card .state=${s}></hc-state-card>`
                       )}
                   </div>`}
+
+            <hc-modal .open=${this.modalOpen} heading="Add entity"
+                      @hc-modal-close=${() => (this.modalOpen = false)}>
+                <hc-entity-form
+                    @hc-entity-submit=${(e: Event) => this._onSubmit(e as CustomEvent)}
+                    @hc-entity-cancel=${() => (this.modalOpen = false)}></hc-entity-form>
+                <button slot="footer" class="btn" @click=${() => this._form?.requestCancel()}>Cancel</button>
+                <button slot="footer" class="btn primary" @click=${() => this._form?.requestSubmit()}>Create</button>
+            </hc-modal>
         `;
     }
 }