fix: remove /api/ double-prefix from all frontend API calls

Frontend api.ts baseUrl is already "/api", so paths like
"/api/cases" produced "/api/api/cases". Stripped the redundant
prefix from all component calls. Rewrite destination correctly
adds /api/ back for the Go backend.

frontend/src/app/(app)/fristen/page.tsx

@@ -16,7 +16,7 @@ export default function FristenPage() {
 
   const { data: deadlines } = useQuery({
     queryKey: ["deadlines"],
-    queryFn: () => api.get<Deadline[]>("/api/deadlines"),
+    queryFn: () => api.get<Deadline[]>("/deadlines"),
   });
 
   return (