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/Pdf-viewer/PdfViewer.tsx

@@ -19,7 +19,7 @@ import {
   config,
 } from 'folds';
 import FocusTrap from 'focus-trap-react';
-import FileSaver from 'file-saver';
+import { useSaveFile } from '../../hooks/useSaveFile';
 import * as css from './PdfViewer.css';
 import { AsyncStatus } from '../../hooks/useAsyncCallback';
 import { useZoom } from '../../hooks/useZoom';
@@ -36,6 +36,7 @@ export const PdfViewer = as<'div', PdfViewerProps>(
   ({ className, name, src, requestClose, ...props }, ref) => {
     const containerRef = useRef<HTMLDivElement>(null);
     const scrollRef = useRef<HTMLDivElement>(null);
+    const saveFile = useSaveFile();
     const { zoom, zoomIn, zoomOut, setZoom } = useZoom(0.2);
 
     const [pdfJSState, loadPdfJS] = usePdfJSLoader();
@@ -76,7 +77,7 @@ export const PdfViewer = as<'div', PdfViewerProps>(
     }, [docState, pageNo, zoom]);
 
     const handleDownload = () => {
-      FileSaver.saveAs(src, name);
+      saveFile(src, name);
     };
 
     const handleJumpSubmit: FormEventHandler<HTMLFormElement> = (evt) => {