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.ts

@@ -0,0 +1,122 @@
+import { getDataTransferFiles, renameFile } from './dom';
+
+// Guard against pathological drops (deeply nested / huge trees) that could
+// otherwise queue thousands of uploads and freeze the composer.
+export const MAX_DROPPED_FILES = 500;
+
+/**
+ * Synchronously collect the `FileSystemEntry` objects for every item in a
+ * drop's `DataTransfer`.
+ *
+ * This MUST be called synchronously inside the drop event handler: the
+ * `DataTransferItemList` is emptied once the handler returns, so calling
+ * `webkitGetAsEntry()` after an `await` yields `null`. Capture the entries
+ * first, then traverse them asynchronously with {@link filesFromEntries}.
+ *
+ * Returns an empty array when `webkitGetAsEntry` is unavailable (non-Chromium
+ * browsers), signalling the caller to fall back to the flat file list.
+ */
+export const entriesFromDataTransfer = (dataTransfer: DataTransfer): FileSystemEntry[] => {
+  const entries: FileSystemEntry[] = [];
+  const { items } = dataTransfer;
+  if (!items) return entries;
+
+  for (let i = 0; i < items.length; i += 1) {
+    const item = items[i];
+    if (item && item.kind === 'file' && typeof item.webkitGetAsEntry === 'function') {
+      const entry = item.webkitGetAsEntry();
+      if (entry) entries.push(entry);
+    }
+  }
+
+  return entries;
+};
+
+const fileFromFileEntry = (entry: FileSystemFileEntry): Promise<File> =>
+  new Promise((resolve, reject) => {
+    entry.file(resolve, reject);
+  });
+
+/**
+ * Read every entry from a directory reader.
+ *
+ * `readEntries` returns results in BATCHES (Chromium yields at most ~100 per
+ * call), so it must be called repeatedly until it resolves with an empty array.
+ */
+const readAllDirectoryEntries = (reader: FileSystemDirectoryReader): Promise<FileSystemEntry[]> =>
+  new Promise((resolve, reject) => {
+    const all: FileSystemEntry[] = [];
+    const readBatch = () => {
+      reader.readEntries((batch) => {
+        if (batch.length === 0) {
+          resolve(all);
+          return;
+        }
+        all.push(...batch);
+        readBatch();
+      }, reject);
+    };
+    readBatch();
+  });
+
+/**
+ * Recursively walk `FileSystemEntry` objects (as produced by
+ * {@link entriesFromDataTransfer}) and resolve them into a flat `File[]`,
+ * descending into every nested directory.
+ *
+ * Nested files keep their relative folder path as a name prefix (e.g.
+ * `photos/2024/pic.jpg`) so uploads remain distinguishable. Traversal stops
+ * once `maxFiles` files have been collected.
+ */
+export const filesFromEntries = async (
+  entries: FileSystemEntry[],
+  maxFiles: number = MAX_DROPPED_FILES,
+): Promise<File[]> => {
+  const files: File[] = [];
+
+  const walk = async (entry: FileSystemEntry, prefix: string): Promise<void> => {
+    if (files.length >= maxFiles) return;
+
+    if (entry.isFile) {
+      const file = await fileFromFileEntry(entry as FileSystemFileEntry);
+      if (files.length >= maxFiles) return;
+      files.push(prefix ? renameFile(file, `${prefix}${file.name}`) : file);
+      return;
+    }
+
+    if (entry.isDirectory) {
+      const reader = (entry as FileSystemDirectoryEntry).createReader();
+      const childEntries = await readAllDirectoryEntries(reader);
+      const childPrefix = `${prefix}${entry.name}/`;
+      for (const child of childEntries) {
+        if (files.length >= maxFiles) break;
+        // eslint-disable-next-line no-await-in-loop
+        await walk(child, childPrefix);
+      }
+    }
+  };
+
+  for (const entry of entries) {
+    if (files.length >= maxFiles) break;
+    // eslint-disable-next-line no-await-in-loop
+    await walk(entry, '');
+  }
+
+  return files;
+};
+
+/**
+ * Extract dropped files, descending into any dropped folders.
+ *
+ * Captures the `FileSystemEntry` list synchronously (required — see
+ * {@link entriesFromDataTransfer}) then traverses it asynchronously. Falls back
+ * to the flat `dataTransfer.files` list when the directory API is unavailable
+ * (non-Chromium) or when no entries are exposed.
+ */
+export const collectDroppedFiles = (dataTransfer: DataTransfer): Promise<File[] | undefined> => {
+  const entries = entriesFromDataTransfer(dataTransfer);
+  if (entries.length === 0) {
+    return Promise.resolve(getDataTransferFiles(dataTransfer));
+  }
+  return filesFromEntries(entries).then((files) => (files.length > 0 ? files : undefined));
+};