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

frontend/lib/api.ts

@@ -1,9 +1,20 @@
 import type { CityDetail, CitySummary } from "@/lib/types";
 
+async function readError(res: Response, fallback: string) {
+  try {
+    const data = await res.json();
+    const msg = data?.error ? String(data.error) : fallback;
+    const detail = data?.detail ? ` ${String(data.detail)}` : "";
+    return `${msg}${detail}`.trim();
+  } catch {
+    return `${fallback}: ${res.status}`;
+  }
+}
+
 export async function getCities(): Promise<CitySummary[]> {
   const res = await fetch("/api/cities", { cache: "no-store" });
   if (!res.ok) {
-    throw new Error(`Failed to load cities: ${res.status}`);
+    throw new Error(await readError(res, `Failed to load cities (${res.status})`));
   }
   const data = await res.json();
   return data.cities ?? [];
@@ -21,7 +32,9 @@ export async function getCityDetail(
     },
   );
   if (!res.ok) {
-    throw new Error(`Failed to load city detail: ${res.status}`);
+    throw new Error(
+      await readError(res, `Failed to load city detail (${res.status})`),
+    );
   }
   return await res.json();
 }