feat(download): show a toast + button check when a file is saved

The desktop (Tauri) app has no native download UI, so FileSaver.saveAs saved
files silently — no visual or audio confirmation. Users re-clicked because
nothing said it worked (one report: 5 copies of the same file). Add a small
useSaveFile() hook that saves AND raises a 'Downloaded <filename>' toast, and
route every download call site through it (file attachments, image viewer, PDF
viewer, plus the recovery-key / key-backup exports). The file-message download
button also shows a green check on success.

Toast system extended with an optional iconSrc so system toasts render an icon
instead of an avatar/initials, and an empty roomName is no longer rendered.

Tests: createDownloadToast covered; 701/701 pass; typecheck + build clean.

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

src/app/hooks/useSaveFile.ts

@@ -0,0 +1,24 @@
+import { useCallback } from 'react';
+import { useSetAtom } from 'jotai';
+import { Icons } from 'folds';
+import FileSaver from 'file-saver';
+import { createDownloadToast, toastQueueAtom } from '../state/toast';
+
+/**
+ * Save a blob/URL to disk AND surface a "Downloaded <filename>" toast.
+ *
+ * The desktop (Tauri) app has no native download UI, so `FileSaver.saveAs` saved
+ * files silently — users re-clicked because nothing confirmed success. This gives
+ * uniform, visible feedback across web + desktop for every download call site.
+ */
+export const useSaveFile = () => {
+  const setToast = useSetAtom(toastQueueAtom);
+
+  return useCallback(
+    (data: Blob | string, filename: string) => {
+      FileSaver.saveAs(data, filename);
+      setToast(createDownloadToast(filename, Icons.Check));
+    },
+    [setToast],
+  );
+};