feat(desktop): app shell — rail, chat panel, routing, states (#15)

Reimplements the handoff Shell in React/TS: 208px left rail (nav +
connection dot + Evening service theme switch), max-1120 main column,
330px Reginald chat panel. Routing across top-level views with an
issue drill-in + back-stack of one; data-theme owned by the shell.

- ChatPanel: fixture write-path — echoes a canned reply so layout +
  interactions are real; model router wiring lands in P4.
- states.tsx: EmptyState / OfflineBanner / ModelAwayState + the States
  specimen gallery, ported from the handoff.
- PlaceholderScreen stands in for not-yet-built views (P3-3+), keeping
  navigation live; it also exposes the issue drill-in for now.
- Gallery loses its own theme toggle (shell owns data-theme); reachable
  via a Primitives rail entry as a living reference.

Fixtures mirrored from the handoff's data.js. typecheck + 7 e2e green
(nav, theme, offline banner + disabled composer, chat echo, drill-in
back-stack); light/dark/states screenshots verified.

Closes P3-2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/desktop/e2e/pages/app.page.ts

@@ -15,13 +15,19 @@ const SCREENS_DIR = join(here, '..', '.artifacts', 'screens')
 export class AppPage {
   constructor(readonly page: Page) {}
 
-  get heading(): Locator {
-    return this.page.getByRole('heading', { name: 'CommiTea' })
+  /** The rail wordmark — present on every in-app view. */
+  get wordmark(): Locator {
+    return this.page.getByText('CommiTea', { exact: true })
+  }
+
+  /** Navigate via a left-rail entry by its label. */
+  nav(label: string): Locator {
+    return this.page.getByRole('button', { name: label, exact: true })
   }
 
   /** Assert the shell has rendered. */
   async expectLoaded(): Promise<void> {
-    await expect(this.heading).toBeVisible()
+    await expect(this.wordmark).toBeVisible()
   }
 
   /** Read the `commitea` preload API surface from the renderer. */