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/FileHeader.tsx

@@ -1,8 +1,8 @@
 import { Badge, Box, Icon, IconButton, Icons, Spinner, Text, as, toRem } from 'folds';
 import React, { ReactNode, useCallback } from 'react';
 import { EncryptedAttachmentInfo } from 'browser-encrypt-attachment';
-import FileSaver from 'file-saver';
 import { mimeTypeToExt } from '../../utils/mimeTypes';
+import { useSaveFile } from '../../hooks/useSaveFile';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
 import { AsyncStatus, useAsyncCallback } from '../../hooks/useAsyncCallback';
@@ -24,6 +24,7 @@ type FileDownloadButtonProps = {
 export function FileDownloadButton({ filename, url, mimeType, encInfo }: FileDownloadButtonProps) {
   const mx = useMatrixClient();
   const useAuthentication = useMediaAuthentication();
+  const saveFile = useSaveFile();
 
   const [downloadState, download] = useAsyncCallback(
     useCallback(async () => {
@@ -34,18 +35,19 @@ export function FileDownloadButton({ filename, url, mimeType, encInfo }: FileDow
         : await downloadMedia(mediaUrl);
 
       const fileURL = URL.createObjectURL(fileContent);
-      FileSaver.saveAs(fileURL, filename);
+      saveFile(fileURL, filename);
       return fileURL;
-    }, [mx, url, useAuthentication, mimeType, encInfo, filename]),
+    }, [mx, url, useAuthentication, mimeType, encInfo, filename, saveFile]),
   );
 
   const downloading = downloadState.status === AsyncStatus.Loading;
   const hasError = downloadState.status === AsyncStatus.Error;
+  const succeeded = downloadState.status === AsyncStatus.Success;
   return (
     <IconButton
       disabled={downloading}
       onClick={download}
-      variant={hasError ? 'Critical' : 'SurfaceVariant'}
+      variant={hasError ? 'Critical' : succeeded ? 'Success' : 'SurfaceVariant'}
       size="300"
       radii="300"
       aria-label={
@@ -53,13 +55,15 @@ export function FileDownloadButton({ filename, url, mimeType, encInfo }: FileDow
           ? 'Downloading...'
           : hasError
             ? 'Download failed, click to retry'
-            : 'Download file'
+            : succeeded
+              ? 'Downloaded — click to download again'
+              : 'Download file'
       }
     >
       {downloading ? (
         <Spinner size="100" variant={hasError ? 'Critical' : 'Secondary'} />
       ) : (
-        <Icon size="100" src={Icons.Download} />
+        <Icon size="100" src={succeeded ? Icons.Check : Icons.Download} />
       )}
     </IconButton>
   );