Add cached dashboard prewarm hints for priority cities

frontend/hooks/useDashboardStore.tsx

@@ -618,6 +618,25 @@ export function DashboardStoreProvider({
     void loadCities();
   }, []);
 
+  useEffect(() => {
+    if (!cities.length) return;
+    if (typeof window === "undefined") return;
+    const schedule = () => dashboardClient.sendPriorityWarmHint();
+    const idleCallback = (window as Window & {
+      requestIdleCallback?: (callback: () => void, options?: { timeout: number }) => number;
+    }).requestIdleCallback;
+    if (typeof idleCallback === "function") {
+      const id = idleCallback(schedule, { timeout: 3000 });
+      return () => {
+        if (typeof window.cancelIdleCallback === "function") {
+          window.cancelIdleCallback(id);
+        }
+      };
+    }
+    const timer = window.setTimeout(schedule, 2000);
+    return () => window.clearTimeout(timer);
+  }, [cities.length]);
+
   useEffect(() => {
     void refreshProAccess();
   }, []);

frontend/lib/dashboard-client.ts

@@ -12,6 +12,7 @@ const CACHE_TTL_MS = 5 * 60 * 1000;
 const pendingCityDetailRequests = new Map<string, Promise<CityDetail>>();
 const pendingHistoryRequests = new Map<string, Promise<HistoryPayload>>();
 const pendingCitySummaryRequests = new Map<string, Promise<CitySummary>>();
+const PRIORITY_WARM_SESSION_KEY = "polyWeather_priority_warm_v1";
 
 type CityCacheMeta = {
   cachedAt: number;
@@ -142,6 +143,24 @@ export const dashboardClient = {
     return data.cities || [];
   },
 
+  sendPriorityWarmHint(timezone?: string | null) {
+    if (!isClient()) return;
+    const tz = String(
+      timezone || Intl.DateTimeFormat().resolvedOptions().timeZone || "",
+    ).trim();
+    if (!tz) return;
+    const cacheKey = `${PRIORITY_WARM_SESSION_KEY}:${tz}`;
+    if (window.sessionStorage.getItem(cacheKey)) return;
+    window.sessionStorage.setItem(cacheKey, "1");
+    const params = new URLSearchParams({ timezone: tz });
+    void fetch(`/api/system/priority-warm?${params.toString()}`, {
+      method: "POST",
+      headers: { Accept: "application/json" },
+      cache: "no-store",
+      keepalive: true,
+    }).catch(() => {});
+  },
+
   async getCitySummary(cityName: string, options?: { force?: boolean }) {
     const force = options?.force ?? false;
     const requestKey = `${cityName}::${force ? "force" : "cached"}`;