fix: image upload proxy, compression, and JSON parse safety

- Add /api/v1/ and /files/ rewrite rules in next.config.ts so frontend
  can call backend without /admin139 prefix
- Fix upload using MultiPartParser instead of req->getUploadedFiles()
- Add client-side image compression (canvas resize to 1920px, quality 0.8)
  for photos >500KB before upload
- Safe JSON parsing: catch HTML error responses instead of throwing
  SyntaxError on non-JSON backend responses
- Fix backslash escape in delete filename validation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/next.config.ts

@@ -18,6 +18,16 @@ const nextConfig: NextConfig = {
         source: "/admin139/:path+",
         destination: `${backendInternal}/:path*`,
       },
+      {
+        // Proxy backend API calls made without /admin139 prefix
+        source: "/api/v1/:path*",
+        destination: `${backendInternal}/api/v1/:path*`,
+      },
+      {
+        // Proxy backend static files (note images etc.)
+        source: "/files/:path*",
+        destination: `${backendInternal}/files/:path*`,
+      },
     ];
   },
 };