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/state/toast.test.ts

@@ -1,7 +1,7 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { createStore } from 'jotai';
-import { toastQueueAtom, dismissToastAtom, ToastNotif } from './toast';
+import { toastQueueAtom, dismissToastAtom, ToastNotif, createDownloadToast } from './toast';
 
 // The queue lives in an unexported baseAtom; we drive the two write-only setters
 // (toastQueueAtom append + null no-op guard, dismissToastAtom remove-by-id)
@@ -85,3 +85,15 @@ test('dismissToastAtom for an unknown id is a no-op', () => {
     ['a'],
   );
 });
+
+test('createDownloadToast: filename in body, no room navigation, unique ids', () => {
+  const a = createDownloadToast('photo.jpg');
+  assert.equal(a.displayName, 'Downloaded');
+  assert.equal(a.body, 'photo.jpg');
+  // roomId empty + an onClick present → clicking dismisses without navigating to a room.
+  assert.equal(a.roomId, '');
+  assert.equal(a.roomName, '');
+  assert.equal(typeof a.onClick, 'function');
+  const b = createDownloadToast('photo.jpg');
+  assert.notEqual(a.id, b.id);
+});