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/utils/saveMedia.ts

@@ -0,0 +1,55 @@
+import FileSaver from 'file-saver';
+import { downloadMedia } from './matrix';
+import { getCurrentAccessToken } from './auth';
+
+export type MediaSaver = (blob: Blob, filename: string) => void | Promise<void>;
+
+let customMediaSaver: MediaSaver | null = null;
+
+const defaultMediaSaver: MediaSaver = (blob, filename) => {
+  FileSaver.saveAs(blob, filename);
+};
+
+/**
+ * Register a platform-specific media saver (Android MediaStore, Electron dialog, etc.).
+ * Pass null to restore the default FileSaver path.
+ *
+ * Platforms should call this once at startup (mobile overlay / desktop shell entry).
+ */
+export const registerMediaSaver = (saver: MediaSaver | null): void => {
+  customMediaSaver = saver;
+};
+
+/**
+ * Persist a Blob to disk via the registered platform saver, or FileSaver by default.
+ */
+export const saveMediaBlob = async (blob: Blob, filename: string): Promise<void> => {
+  const saver = customMediaSaver ?? defaultMediaSaver;
+  await saver(blob, filename);
+};
+
+/**
+ * Fetch media (auth download or blob:/data:/http URL) and save it via [saveMediaBlob].
+ */
+export const downloadAndSaveMedia = async (
+  src: string,
+  filename: string,
+  accessToken?: string | null
+): Promise<void> => {
+  let blob: Blob;
+  try {
+    if (src.startsWith('blob:') || src.startsWith('data:')) {
+      const res = await fetch(src);
+      if (!res.ok) throw new Error(`Failed to fetch ${src}`);
+      blob = await res.blob();
+    } else {
+      blob = await downloadMedia(src, accessToken ?? getCurrentAccessToken());
+    }
+  } catch (error) {
+    console.warn('[saveMedia] downloadMedia failed, trying fetch fallback:', error);
+    const res = await fetch(src);
+    if (!res.ok) throw error;
+    blob = await res.blob();
+  }
+  await saveMediaBlob(blob, filename);
+};