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/components/DeviceVerificationSetup.tsx

@@ -13,9 +13,9 @@ import {
   color,
   Spinner,
 } from 'folds';
-import FileSaver from 'file-saver';
 import to from 'await-to-js';
 import { AuthDict, IAuthData, MatrixError, UIAuthCallback } from 'matrix-js-sdk';
+import { useSaveFile } from '../hooks/useSaveFile';
 import { useModalStyle } from '../hooks/useModalStyle';
 import { PasswordInput } from './password-input';
 import { ContainerColor } from '../styles/ContainerColor.css';
@@ -230,6 +230,7 @@ type RecoveryKeyDisplayProps = {
 };
 function RecoveryKeyDisplay({ recoveryKey }: RecoveryKeyDisplayProps) {
   const [show, setShow] = useState(false);
+  const saveFile = useSaveFile();
 
   const handleCopy = () => {
     copyToClipboard(recoveryKey);
@@ -239,7 +240,7 @@ function RecoveryKeyDisplay({ recoveryKey }: RecoveryKeyDisplayProps) {
     const blob = new Blob([recoveryKey], {
       type: 'text/plain;charset=us-ascii',
     });
-    FileSaver.saveAs(blob, 'recovery-key.txt');
+    saveFile(blob, 'recovery-key.txt');
   };
 
   const safeToDisplayKey = show ? recoveryKey : recoveryKey.replace(/[^\s]/g, '*');