修复图表时区计算和 stale 缓存展示

- formatCityLocalDate 改为纯 UTC+cityOffset+getUTC*，不再受浏览器时区影响
- 新增 formatCityLocalDateTime 统一更新时间的时区处理
- 缓存过期时静默后台刷新，不转 loading spinner
- showingStaleDetail 现在不依赖 detailError 就显示橙色标签

frontend/components/dashboard/scan-terminal/LiveTemperatureThresholdChart.tsx

@@ -103,13 +103,27 @@ function peakGlowTitle(
 
 function formatCityLocalDate(tzOffsetSeconds: number | null | undefined) {
   const cityOffsetMs = (tzOffsetSeconds ?? 0) * 1000;
-  const cityNow = new Date(Date.now() + cityOffsetMs + new Date().getTimezoneOffset() * 60_000);
+  const cityNow = new Date(Date.now() + cityOffsetMs);
-  const y = cityNow.getFullYear();
+  const y = cityNow.getUTCFullYear();
-  const m = String(cityNow.getMonth() + 1).padStart(2, "0");
+  const m = String(cityNow.getUTCMonth() + 1).padStart(2, "0");
-  const d = String(cityNow.getDate()).padStart(2, "0");
+  const d = String(cityNow.getUTCDate()).padStart(2, "0");
   return `${y}-${m}-${d}`;
 }
 
+function formatCityLocalDateTime(tzOffsetSeconds: number | null | undefined) {
+  const nowUtc = Date.now();
+  const cityOffsetMs = (tzOffsetSeconds ?? 0) * 1000;
+  const cityNow = new Date(nowUtc + cityOffsetMs);
+  const pad = (n: number) => String(n).padStart(2, "0");
+  const y = cityNow.getUTCFullYear();
+  const mo = pad(cityNow.getUTCMonth() + 1);
+  const d = pad(cityNow.getUTCDate());
+  const hh = pad(cityNow.getUTCHours());
+  const mm = pad(cityNow.getUTCMinutes());
+  const ss = pad(cityNow.getUTCSeconds());
+  return `${y}-${mo}-${d} ${hh}:${mm}:${ss}`;
+}
+
 function getLiveTempFromHourly(data: HourlyForecast) {
   return validNumber(data?.airportCurrent?.temp) ?? validNumber(data?.airportPrimary?.temp) ?? null;
 }
@@ -878,6 +892,9 @@ export function LiveTemperatureThresholdChart({
       return;
     }
 
+    // ── Stale-while-revalidate: show cached data, refresh in background ──
+    const hasStaleCache = cached && !hasFreshCache;
+
     if (
       !shouldFetchCityDetailForChart({
         city,
@@ -899,7 +916,11 @@ export function LiveTemperatureThresholdChart({
       commitHourlySnapshot((prev) => mergeRowObservationIntoHourly(prev, getLatestRowSnapshot()));
       setShowingStaleDetail(false);
     }
-    setIsHourlyLoading(true);
+
+    // Stale cache: don't show loading spinner, refresh silently
+    if (!hasStaleCache) {
+      setIsHourlyLoading(true);
+    }
     let cancelled = false;
     let retryScheduled = false;
     let retryTimer: ReturnType<typeof setTimeout> | null = null;
@@ -1292,17 +1313,7 @@ export function LiveTemperatureThresholdChart({
   );
 
   const formattedUpdateTime = useMemo(() => {
-    const nowUtc = Date.now();
+    return formatCityLocalDateTime(row?.tz_offset_seconds);
-    const cityOffsetMs = (row?.tz_offset_seconds ?? 0) * 1000;
-    const cityNow = new Date(nowUtc + cityOffsetMs + new Date().getTimezoneOffset() * 60_000);
-    const pad = (n: number) => String(n).padStart(2, "0");
-    const y = cityNow.getFullYear();
-    const mo = pad(cityNow.getMonth() + 1);
-    const d = pad(cityNow.getDate());
-    const hh = pad(cityNow.getHours());
-    const mm = pad(cityNow.getMinutes());
-    const ss = pad(cityNow.getSeconds());
-    return `${y}-${mo}-${d} ${hh}:${mm}:${ss}`;
   }, [row]);
 
   const cityThresholds = useMemo(() => {
@@ -1692,6 +1703,8 @@ export const __getLiveObservationLabelsForTest = getLiveObservationLabels;
 export const __getObservationDisplayMetricsForTest = getObservationDisplayMetrics;
 export const __getPeakGlowStateForTest = getPeakGlowState;
 export const __getWundergroundDailyHighForTest = getWundergroundDailyHigh;
+export const __formatCityLocalDateForTest = formatCityLocalDate;
+export const __formatCityLocalDateTimeForTest = formatCityLocalDateTime;
 export const __getInitialDetailLoadDelayMsForTest = getInitialDetailLoadDelayMs;
 export const __shouldFetchCityDetailForChartForTest = shouldFetchCityDetailForChart;
 export const __shouldPollLiveChartForTest = shouldPollLiveChart;

frontend/components/dashboard/scan-terminal/TemperatureChartCanvas.tsx

@@ -228,7 +228,8 @@ function TemperatureChartCanvasComponent({
   const shouldShowBackgroundRefresh = isHourlyLoading && hasDrawableChartContent;
   const shouldShowUnavailableState = Boolean(row?.city) && Boolean(detailError) && !isHourlyLoading && !hasDrawableChartContent;
   const shouldShowBackgroundError =
-    showDetailErrorBadge && Boolean(row?.city) && Boolean(detailError) && !isHourlyLoading && hasDrawableChartContent;
+    showDetailErrorBadge && Boolean(row?.city) && !isHourlyLoading && hasDrawableChartContent &&
+    (Boolean(detailError) || showingStaleDetail || detailStatus === "stale_cache");
   const backgroundErrorLabel =
     showingStaleDetail || detailStatus === "stale_cache"
       ? (isEn ? "Detail cache" : "详情缓存")

frontend/components/dashboard/scan-terminal/__tests__/temperatureDefaultVisibilityPolicy.test.ts

@@ -7,6 +7,8 @@ import {
   __getPeakGlowStateForTest,
   __getWundergroundDailyHighForTest,
   __getVisibleTemperatureSeriesForTest,
+  __formatCityLocalDateForTest,
+  __formatCityLocalDateTimeForTest,
   __isTemperatureSeriesVisibleByDefaultForTest,
   __mergePatchIntoHourlyForTest,
   __selectCompactSecondaryTempForTest,
@@ -27,6 +29,39 @@ function runwayKey(rwy: string) {
 }
 
 export function runTests() {
+  {
+    const originalDateNow = Date.now;
+    const originalGetTimezoneOffsetForCityDate = Date.prototype.getTimezoneOffset;
+    try {
+      Date.now = () => Date.UTC(2026, 5, 15, 14, 0, 0);
+      Date.prototype.getTimezoneOffset = function () {
+        return 0;
+      };
+      assert(
+        __formatCityLocalDateForTest(9 * 60 * 60) === "2026-06-15",
+        "Tokyo local date should be formatted from UTC plus city offset, independent of browser timezone",
+      );
+      assert(
+        __formatCityLocalDateTimeForTest(9 * 60 * 60) === "2026-06-15 23:00:00",
+        "Tokyo update time should be formatted from UTC plus city offset, independent of browser timezone",
+      );
+      Date.prototype.getTimezoneOffset = function () {
+        return -8 * 60;
+      };
+      assert(
+        __formatCityLocalDateForTest(9 * 60 * 60) === "2026-06-15",
+        "Tokyo local date should not change when the browser timezone changes",
+      );
+      assert(
+        __formatCityLocalDateTimeForTest(9 * 60 * 60) === "2026-06-15 23:00:00",
+        "Tokyo update time should not change when the browser timezone changes",
+      );
+    } finally {
+      Date.now = originalDateNow;
+      Date.prototype.getTimezoneOffset = originalGetTimezoneOffsetForCityDate;
+    }
+  }
+
   const peakGlowSeries = [
     {
       key: "madis",