fix: throw on non-OK response in downloadMedia (Fixes JAVASCRIPT-REACT-B)

When the server returns a 4xx/5xx, downloadMedia was silently returning
the error response body as a blob. decryptAttachment would then fail with
a misleading 'Mismatched SHA-256 digest' instead of surfacing the real
HTTP error. Now throws immediately so callers (useAsyncCallback) can
show the correct error state.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/utils/matrix.ts

@@ -299,6 +299,7 @@ export const mxcUrlToHttp = (
 export const downloadMedia = async (src: string): Promise<Blob> => {
   // this request is authenticated by service worker
   const res = await fetch(src, { method: 'GET' });
+  if (!res.ok) throw new Error(`Media download failed: ${res.status} ${res.statusText}`);
   const blob = await res.blob();
   return blob;
 };