feat: Implement HTTP caching for API routes and add a dashboard sidebar with city grouping and state persistence.

frontend/app/api/cities/route.ts

@@ -1,10 +1,11 @@
-import { NextResponse } from "next/server";
+import { NextRequest, NextResponse } from "next/server";
 import { buildBackendRequestHeaders } from "@/lib/backend-auth";
+import { buildCachedJsonResponse } from "@/lib/http-cache";
 
 const API_BASE = process.env.POLYWEATHER_API_BASE_URL;
 export const dynamic = "force-dynamic";
 
-export async function GET() {
+export async function GET(req: NextRequest) {
   if (!API_BASE) {
     return NextResponse.json(
       { error: "POLYWEATHER_API_BASE_URL is not configured" },
@@ -25,7 +26,11 @@ export async function GET() {
       );
     }
     const data = await res.json();
-    return NextResponse.json(data);
+    return buildCachedJsonResponse(
+      req,
+      data,
+      "public, max-age=0, s-maxage=300, stale-while-revalidate=1800",
+    );
   } catch (error) {
     return NextResponse.json(
       { error: "Failed to fetch cities", detail: String(error) },