feat: Implement initial PolyWeather dashboard with city list, detail view, map, and API endpoints.

frontend/app/api/cities/route.ts

@@ -1,17 +1,24 @@
 import { NextResponse } from "next/server";
 
-const API_BASE =
-  process.env.POLYWEATHER_API_BASE_URL || "http://127.0.0.1:8000";
+const API_BASE = process.env.POLYWEATHER_API_BASE_URL;
 
 export async function GET() {
+  if (!API_BASE) {
+    return NextResponse.json(
+      { error: "POLYWEATHER_API_BASE_URL is not configured" },
+      { status: 500 },
+    );
+  }
+
   try {
     const res = await fetch(`${API_BASE}/api/cities`, {
       headers: { Accept: "application/json" },
       next: { revalidate: 120 },
     });
     if (!res.ok) {
+      const raw = await res.text();
       return NextResponse.json(
-        { error: `Backend returned ${res.status}` },
+        { error: `Backend returned ${res.status}`, detail: raw.slice(0, 300) },
         { status: 502 },
       );
     }