feat(desktop): Tier B web side — toast actions, Focus Assist gate, folder DnD

- P5-41/35 useTauriToastActions: native rich-toast click → navigate(path) (opens
  the room), quick reply → mx.sendMessage(roomId, m.text). The desktop bridge
  routes message notifications (tag=roomId) to show_rich_toast.
- P5-56 useTauriFocusAssist + focusAssistActiveAtom: a native focus-assist-changed
  event drives the atom, OR'd into the existing quiet-hours gate in
  ClientNonUIFeatures so notifications+sounds suppress during Windows Focus Assist.
- P5-48 recursive folder drag-drop: fileEntries.ts (sync webkitGetAsEntry capture
  → async batched readEntries traversal, path-prefixed names, 500-file cap) wired
  into useFileDrop, reusing the existing upload pipeline. +3 unit tests.

Hooks no-op in the browser. Gates: tsc/eslint/prettier clean, build OK, 559 tests.

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

src/app/hooks/useTauriFocusAssist.ts

@@ -0,0 +1,24 @@
+import { useSetAtom } from 'jotai';
+import { focusAssistActiveAtom } from '../state/focusAssist';
+import { useTauriEvent } from './useTauri';
+
+/** Detail shape of the `focus-assist-changed` event emitted by the native side. */
+type FocusAssistChangedDetail = {
+  active: boolean;
+};
+
+/**
+ * P5-56 — Windows Focus Assist ↔ Do-Not-Disturb sync (desktop). Subscribes to
+ * the native `focus-assist-changed` event (Windows `SHQueryUserNotificationState`
+ * poll, `{ active }`) and mirrors it into `focusAssistActiveAtom`, which the
+ * notification gate reads to suppress notifications while the shell is in Focus
+ * Assist / Quiet Hours, presenting, gaming full-screen, or busy. Inert in the
+ * browser, since `useTauriEvent` only listens under Tauri.
+ */
+export function useTauriFocusAssist(): void {
+  const setFocusAssist = useSetAtom(focusAssistActiveAtom);
+
+  useTauriEvent<FocusAssistChangedDetail>('focus-assist-changed', ({ active }) =>
+    setFocusAssist(active),
+  );
+}