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/message/content/FileContent.tsx

@@ -14,10 +14,10 @@ import {
   TooltipProvider,
   as,
 } from 'folds';
-import FileSaver from 'file-saver';
 import { EncryptedAttachmentInfo } from 'browser-encrypt-attachment';
 import FocusTrap from 'focus-trap-react';
 import { IFileInfo } from '../../../../types/matrix/common';
+import { useSaveFile } from '../../../hooks/useSaveFile';
 import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { bytesToSize } from '../../../utils/common';
@@ -252,6 +252,7 @@ export type DownloadFileProps = {
 export function DownloadFile({ body, mimeType, url, info, encInfo }: DownloadFileProps) {
   const mx = useMatrixClient();
   const useAuthentication = useMediaAuthentication();
+  const saveFile = useSaveFile();
 
   const [downloadState, download] = useAsyncCallback(
     useCallback(async () => {
@@ -262,9 +263,9 @@ export function DownloadFile({ body, mimeType, url, info, encInfo }: DownloadFil
         : await downloadMedia(mediaUrl);
 
       const fileURL = URL.createObjectURL(fileContent);
-      FileSaver.saveAs(fileURL, body);
+      saveFile(fileURL, body);
       return fileURL;
-    }, [mx, url, useAuthentication, mimeType, encInfo, body]),
+    }, [mx, url, useAuthentication, mimeType, encInfo, body, saveFile]),
   );
 
   return downloadState.status === AsyncStatus.Error ? (
@@ -277,7 +278,7 @@ export function DownloadFile({ body, mimeType, url, info, encInfo }: DownloadFil
       size="400"
       onClick={() =>
         downloadState.status === AsyncStatus.Success
-          ? FileSaver.saveAs(downloadState.data, body)
+          ? saveFile(downloadState.data, body)
           : download()
       }
       disabled={downloadState.status === AsyncStatus.Loading}