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/features/settings/devices/LocalBackup.tsx

@@ -1,6 +1,6 @@
 import React, { FormEventHandler, useCallback, useEffect, useState } from 'react';
 import { Box, Button, color, Icon, Icons, Spinner, Text, toRem } from 'folds';
-import FileSaver from 'file-saver';
+import { useSaveFile } from '../../../hooks/useSaveFile';
 import { SequenceCard } from '../../../components/sequence-card';
 import { SettingTile } from '../../../components/setting-tile';
 import { SequenceCardStyle } from '../styles.css';
@@ -15,6 +15,7 @@ import { useFilePicker } from '../../../hooks/useFilePicker';
 function ExportKeys() {
   const mx = useMatrixClient();
   const alive = useAlive();
+  const saveFile = useSaveFile();
 
   const [exportState, exportKeys] = useAsyncCallback<void, Error, [string]>(
     useCallback(
@@ -28,9 +29,9 @@ function ExportKeys() {
         const blob = new Blob([encKeys], {
           type: 'text/plain;charset=us-ascii',
         });
-        FileSaver.saveAs(blob, 'lotus-keys.txt');
+        saveFile(blob, 'lotus-keys.txt');
       },
-      [mx],
+      [mx, saveFile],
     ),
   );