feat(desktop): tray Do-Not-Disturb + Launch-on-login toggle (P6-1 web)

- useTauriDnd + manualDndAtom: the native tray "Do Not Disturb" toggle
  (lotus-dnd-changed event) OR's into the notification quiet-gate in
  ClientNonUIFeatures (both invite + message notifiers), alongside Focus Assist.
- AutostartSetting in Settings → General (desktop-only): reads/sets
  plugin:autostart via invoke. Mirrors the window-chrome setting.
- Docs: LOTUS_FEATURES desktop section (Linux parity + DND + autostart),
  LOTUS_TODO P6-1 → [~], LOTUS_BUGS verification row.

Gates: tsc/eslint/prettier clean, build OK, 661 tests. Native side committed on
cinny-desktop:main (CI-compile-pending).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/TauriDesktopFeatures.tsx

@@ -5,6 +5,7 @@ import { useTauriSmtc } from '../hooks/useTauriSmtc';
 import { useTauriNetwork } from '../hooks/useTauriNetwork';
 import { useTauriToastActions } from '../hooks/useTauriToastActions';
 import { useTauriFocusAssist } from '../hooks/useTauriFocusAssist';
+import { useTauriDnd } from '../hooks/useTauriDnd';
 
 /**
  * Mounts the client-scoped native desktop feature hooks (call/room aware). Each
@@ -21,5 +22,6 @@ export function TauriDesktopFeatures(): null {
   useTauriNetwork(); // P5-49 network-change awareness → sync retry
   useTauriToastActions(); // P5-41/35 rich toast click → open room, quick reply → send
   useTauriFocusAssist(); // P5-56 Windows Focus Assist → DND suppression atom
+  useTauriDnd(); // P6-1 tray "Do Not Disturb" → notification suppression atom
   return null;
 }