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/useFileDrop.ts

@@ -1,11 +1,14 @@
 import { useCallback, DragEventHandler, RefObject, useState, useEffect, useRef } from 'react';
-import { getDataTransferFiles } from '../utils/dom';
+import { collectDroppedFiles } from '../utils/fileEntries';
 
 export const useFileDropHandler = (onDrop: (file: File[]) => void): DragEventHandler =>
   useCallback(
     (evt) => {
-      const files = getDataTransferFiles(evt.dataTransfer);
-      if (files) onDrop(files);
+      // `collectDroppedFiles` synchronously captures the entry list from the
+      // DataTransfer before traversing folders asynchronously.
+      collectDroppedFiles(evt.dataTransfer).then((files) => {
+        if (files) onDrop(files);
+      });
     },
     [onDrop],
   );
@@ -24,8 +27,12 @@ export const useFileDropZone = (
       dragCounterRef.current = 0;
       setActive(false);
       if (!evt.dataTransfer) return;
-      const files = getDataTransferFiles(evt.dataTransfer);
-      if (files) onDrop(files);
+      // Capture entries synchronously (inside the event) then traverse any
+      // dropped folders asynchronously — the DataTransferItemList is emptied
+      // once this handler returns.
+      collectDroppedFiles(evt.dataTransfer).then((files) => {
+        if (files) onDrop(files);
+      });
     };
 
     target?.addEventListener('drop', handleDrop);