fix(frontend): API client uses relative URLs in browser (same-origin) + SSR proxy route

- lib/api.ts: detect browser vs server, use relative URL in browser
- /api/catalog/[...path]: proxy route to forward SSR requests to backend
- .env.local: NEXT_PUBLIC_API_URL preserved for SSR
- Closes FRONTEND-PROXY

project/frontend/src/app/api/catalog/[...path]/route.ts

@@ -0,0 +1,63 @@
+import { NextRequest, NextResponse } from 'next/server';
+
+const API = process.env.NEXT_PUBLIC_API_URL ?? 'http://127.0.0.1:3000';
+
+export async function GET(request: NextRequest) {
+  return handleProxy(request);
+}
+
+export async function POST(request: NextRequest) {
+  return handleProxy(request);
+}
+
+export async function PUT(request: NextRequest) {
+  return handleProxy(request);
+}
+
+export async function PATCH(request: NextRequest) {
+  return handleProxy(request);
+}
+
+export async function DELETE(request: NextRequest) {
+  return handleProxy(request);
+}
+
+async function handleProxy(request: NextRequest): Promise<NextResponse> {
+  // Strip the /api/catalog prefix and forward the rest to the backend
+  const path = request.nextUrl.pathname.replace('/api/catalog/', '');
+  const url = `${API}/${path}${request.nextUrl.search}`;
+
+  try {
+    const headers: Record<string, string> = {
+      accept: 'application/json',
+    };
+    // Forward the cookie header from the incoming request (for auth)
+    const cookie = request.headers.get('cookie');
+    if (cookie) headers.cookie = cookie;
+
+    const init: RequestInit = {
+      headers,
+    };
+    const method = request.method;
+    if (['POST', 'PUT', 'PATCH', 'DELETE'].includes(method)) {
+      init.method = method;
+      init.body = await request.text();
+      const contentType = request.headers.get('content-type');
+      if (contentType) headers['content-type'] = contentType;
+    }
+    const res = await fetch(url, init);
+    const data = await res.json().catch(() => ({}));
+
+    // Forward Set-Cookie header from backend if present
+    const setCookie = res.headers.get('set-cookie');
+    if (setCookie) {
+      return NextResponse.json(data, { status: res.status, headers: { 'set-cookie': setCookie } });
+    }
+    return NextResponse.json(data, { status: res.status });
+  } catch (err) {
+    return NextResponse.json(
+      { error: { code: 'PROXY_ERROR', message: String(err) } },
+      { status: 502 }
+    );
+  }
+}

project/frontend/src/lib/api.ts

@@ -8,8 +8,17 @@ interface CmsPage {
   status: string;
 }
 
-// Use env var if set, otherwise use relative URL (works for SSR and client-side via same-origin or CORS)
-const BASE = process.env.NEXT_PUBLIC_API_URL || '';
+// Use env var if set, otherwise proxy through /api/catalog
+// In SSR: NEXT_PUBLIC_API_URL is http://127.0.0.1:3000 (backend direct)
+// In prod (relative mode): empty → /api/catalog (same-origin proxy)
+const BASE = (() => {
+  if (typeof window !== 'undefined') {
+    // Browser: use same-origin relative URL (works from any network)
+    return '';
+  }
+  // Server/SSR: use backend URL from env
+  return process.env.NEXT_PUBLIC_API_URL ?? 'http://127.0.0.1:3000';
+})();
 
 export async function fetchCategories(): Promise<Category[]> {
   const res = await fetch(`${BASE}/categories/tree`);