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/utils/fileEntries.test.ts

@@ -0,0 +1,70 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { filesFromEntries } from './fileEntries';
+
+const fileEntry = (name: string): FileSystemFileEntry =>
+  ({
+    isFile: true,
+    isDirectory: false,
+    name,
+    file: (success: (file: File) => void) => {
+      success(new File(['x'], name, { type: 'text/plain' }));
+    },
+  }) as unknown as FileSystemFileEntry;
+
+/**
+ * A directory whose reader yields its children in several batches (mirroring
+ * Chromium's `readEntries`, which caps each call) and finally an empty batch.
+ */
+const dirEntry = (name: string, children: FileSystemEntry[]): FileSystemDirectoryEntry => {
+  const batches = [children.slice(0, 1), children.slice(1), [] as FileSystemEntry[]];
+  return {
+    isFile: false,
+    isDirectory: true,
+    name,
+    createReader: () => {
+      let call = 0;
+      return {
+        readEntries: (success: (entries: FileSystemEntry[]) => void) => {
+          const batch = batches[call] ?? [];
+          call += 1;
+          success(batch);
+        },
+      } as unknown as FileSystemDirectoryReader;
+    },
+  } as unknown as FileSystemDirectoryEntry;
+};
+
+test('filesFromEntries flattens nested folders and prefixes relative paths', async () => {
+  const entries: FileSystemEntry[] = [
+    fileEntry('top.txt'),
+    dirEntry('photos', [
+      fileEntry('a.jpg'),
+      dirEntry('2024', [fileEntry('b.jpg'), fileEntry('c.jpg')]),
+    ]),
+  ];
+
+  const files = await filesFromEntries(entries);
+  const names = files.map((f) => f.name).sort();
+
+  assert.deepEqual(names, ['photos/2024/b.jpg', 'photos/2024/c.jpg', 'photos/a.jpg', 'top.txt']);
+});
+
+test('filesFromEntries reads directory entries in batches until empty', async () => {
+  const entries: FileSystemEntry[] = [
+    dirEntry('docs', [fileEntry('one.txt'), fileEntry('two.txt')]),
+  ];
+
+  const files = await filesFromEntries(entries);
+  assert.equal(files.length, 2);
+});
+
+test('filesFromEntries respects the maxFiles cap', async () => {
+  const entries: FileSystemEntry[] = [
+    dirEntry('many', [fileEntry('a.txt'), fileEntry('b.txt')]),
+    fileEntry('c.txt'),
+  ];
+
+  const files = await filesFromEntries(entries, 2);
+  assert.equal(files.length, 2);
+});