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/state/manualDnd.ts

@@ -0,0 +1,14 @@
+import { atom } from 'jotai';
+
+/**
+ * P6-1 — Tray "Do Not Disturb" ↔ notification suppression (manual toggle).
+ *
+ * Standalone, non-persisted boolean atom reflecting whether the user has flipped
+ * the native tray "Do Not Disturb" item. It is driven at runtime by
+ * `useTauriDnd` from the native `lotus-dnd-changed` event and read by the
+ * notification gate to suppress notifications while DND is on. Because it mirrors
+ * a transient session toggle — not a persisted user preference — it is a plain
+ * in-memory atom that defaults to `false` and is intentionally NOT written to
+ * `localStorage`.
+ */
+export const manualDndAtom = atom(false);