frontend: skip body in proxy for 204/205/304 responses

Response constructor rejects bodies on these statuses, so DELETE
returning 204 was triggering a 500 in the proxy.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/app/api/proxy/[[...path]]/route.ts

@@ -31,7 +31,9 @@ async function handler(
     body,
   });
 
-  const data = await res.arrayBuffer();
+  // Per Fetch spec, Response with 204/205/304 must not have a body.
+  const hasBody = res.status !== 204 && res.status !== 205 && res.status !== 304;
+  const data = hasBody ? await res.arrayBuffer() : null;
 
   return new NextResponse(data, {
     status: res.status,