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)/ai/extract/page.tsx

@@ -24,7 +24,7 @@ export default function AIExtractPage() {
 
   const { data: casesData } = useQuery({
     queryKey: ["cases"],
-    queryFn: () => api.get<PaginatedResponse<Case>>("/api/cases"),
+    queryFn: () => api.get<PaginatedResponse<Case>>("/cases"),
   });
 
   const cases = casesData?.data ?? [];
@@ -40,12 +40,12 @@ export default function AIExtractPage() {
         const formData = new FormData();
         formData.append("file", file);
         response = await api.postFormData<ExtractionResponse>(
-          "/api/ai/extract-deadlines",
+          "/ai/extract-deadlines",
           formData,
         );
       } else {
         response = await api.post<ExtractionResponse>(
-          "/api/ai/extract-deadlines",
+          "/ai/extract-deadlines",
           { text },
         );
       }
@@ -74,7 +74,7 @@ export default function AIExtractPage() {
 
     try {
       const promises = deadlines.map((d) =>
-        api.post(`/api/cases/${selectedCaseId}/deadlines`, {
+        api.post(`/cases/${selectedCaseId}/deadlines`, {
           title: d.title,
           due_date: d.due_date ?? "",
           source: "ai_extraction",