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/hooks/useTauriDnd.ts

@@ -0,0 +1,21 @@
+import { useSetAtom } from 'jotai';
+import { manualDndAtom } from '../state/manualDnd';
+import { useTauriEvent } from './useTauri';
+
+/** Detail shape of the `lotus-dnd-changed` event emitted by the native side. */
+type DndChangedDetail = {
+  active: boolean;
+};
+
+/**
+ * P6-1 — Tray "Do Not Disturb" → notification suppression (desktop). Subscribes
+ * to the native `lotus-dnd-changed` event (emitted when the user toggles the
+ * tray "Do Not Disturb" item, `{ active }`) and mirrors it into `manualDndAtom`,
+ * which the notification gate reads to suppress notifications while DND is on.
+ * Inert in the browser, since `useTauriEvent` only listens under Tauri.
+ */
+export function useTauriDnd(): void {
+  const setDnd = useSetAtom(manualDndAtom);
+
+  useTauriEvent<DndChangedDetail>('lotus-dnd-changed', ({ active }) => setDnd(active));
+}