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

frontend/lib/http-cache.ts

@@ -0,0 +1,40 @@
+import { createHash } from "node:crypto";
+import { NextRequest, NextResponse } from "next/server";
+
+function normalizeTag(tag: string) {
+  return tag.trim().replace(/^W\//i, "");
+}
+
+function ifNoneMatchHit(ifNoneMatch: string | null, etag: string) {
+  if (!ifNoneMatch) return false;
+  const target = normalizeTag(etag);
+  return ifNoneMatch
+    .split(",")
+    .map((part) => normalizeTag(part))
+    .some((part) => part === "*" || part === target);
+}
+
+function buildEtag(body: string) {
+  const hash = createHash("sha1").update(body).digest("hex");
+  return `"${hash}"`;
+}
+
+export function buildCachedJsonResponse(
+  req: NextRequest,
+  payload: unknown,
+  cacheControl: string,
+) {
+  const body = JSON.stringify(payload);
+  const etag = buildEtag(body);
+  const headers = new Headers({
+    "Cache-Control": cacheControl,
+    ETag: etag,
+    "Content-Type": "application/json; charset=utf-8",
+  });
+
+  if (ifNoneMatchHit(req.headers.get("if-none-match"), etag)) {
+    return new NextResponse(null, { status: 304, headers });
+  }
+
+  return new NextResponse(body, { status: 200, headers });
+}