Add centralized saveMedia helpers for image, video, and file downloads.

Platforms can register a custom saver; viewers share one download/save path instead of FileSaver call sites.

src/app/components/video-viewer/VideoViewer.tsx

@@ -1,10 +1,9 @@
 import React from 'react';
-import FileSaver from 'file-saver';
 import classNames from 'classnames';
 import { Box, Chip, Header, IconButton, Text, as } from 'folds';
 import { Icon, Icons } from '../icons';
 import * as css from './VideoViewer.css';
-import { downloadMedia } from '../../utils/matrix';
+import { downloadAndSaveMedia } from '../../utils/saveMedia';
 import { getCurrentAccessToken } from '../../utils/auth';
 
 export type VideoViewerProps = {
@@ -14,20 +13,16 @@ export type VideoViewerProps = {
 };
 
 export const VideoViewer = as<'div', VideoViewerProps>(
-  ({ className, alt, src, requestClose, ...props }, ref) => {    const handleDownload = async () => {
+  ({ className, alt, src, requestClose, ...props }, ref) => {
+    const handleDownload = async () => {
       try {
-        const fileContent = await downloadMedia(src, getCurrentAccessToken());
-        FileSaver.saveAs(fileContent, alt);
+        await downloadAndSaveMedia(src, alt, getCurrentAccessToken());
       } catch (error) {
         console.warn('[VideoViewer] Failed to download media:', error);
         try {
-          const response = await fetch(src);
-          if (response.ok) {
-            const blob = await response.blob();
-            FileSaver.saveAs(blob, alt);
-          }
-        } catch {
           window.open(src, '_blank');
+        } catch {
+          // ignore
         }
       }
     };