fix: category form — async save with error display, drag-and-drop photos, 20MB body limit

backend/src/app.ts

@@ -31,6 +31,7 @@ export async function buildApp() {
         transport: { target: 'pino-pretty', options: { colorize: true } },
       }),
     },
+    bodyLimit: 20 * 1024 * 1024, // 20MB — for base64 photo uploads
   })
 
   // ── Security ───────────────────────────────────────────────────────────────