feat(settings): toggle gesture control + multi-monitor button (off by default)

Make the two experimental header features opt-in via App Settings instead of
forced on. Both default OFF.

- App Settings → Display → 'Header Displays' gains a 'Multi-monitor Button'
  toggle (setting: showMultiMonitorButton). The button is hidden in the template
  by default; the server reveals it at render when enabled, and
  applyHeaderVisibilitySettings handles live toggles from a save.
- App Settings → Display → new 'Input' section gains a 'Gesture Control' toggle
  (setting: gestureControlEnabled). The gesture overlay is injected at page
  render, so renderIndexHtml (now async) reads settings.json and injects the
  bundle only when enabled; toggling reloads the page. CODEMAN_GESTURE=1 stays
  the instance-level 'feature available' gate (CSP + assets) and exposes
  window.__codemanGestureAvailable so the Input section only shows when usable.
- The retired notification bell stays hidden regardless of notification state.

Both settings added to SettingsUpdateSchema and the mobile defaults.

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

src/web/schemas.ts

@@ -280,6 +280,9 @@ export const SettingsUpdateSchema = z
     showProjectInsights: z.boolean().optional(),
     showFileBrowser: z.boolean().optional(),
     showSubagents: z.boolean().optional(),
+    showMultiMonitorButton: z.boolean().optional(),
+    // Input
+    gestureControlEnabled: z.boolean().optional(),
     // Claude CLI settings
     claudeMode: z.string().max(50).optional(),
     allowedTools: z.string().max(2000).optional(),