新增 v1.5.6 升级公告，3 天后自动消失

公告展示决策终端 hero 重设计、DEB 统一、亮色补全等变更摘要。
localStorage 记录首次访问时间，超过 3 天自动隐藏。
补全 scan-upgrade-announcement 暗色主题基础样式。

Tested: tsc 全过

frontend/components/dashboard/ScanTerminalDashboard.tsx

@@ -115,6 +115,20 @@ function ScanTerminalScreen() {
   const [activeView, setActiveView] = useState<ContentView>("map");
   const [mapSelectedCityName, setMapSelectedCityName] = useState<string | null>(null);
   const [showScanPaywall, setShowScanPaywall] = useState(false);
+  const [showAnnouncement, setShowAnnouncement] = useState(false);
+
+  useEffect(() => {
+    const key = "polyweather_v156_announcement_seen_at";
+    const seen = localStorage.getItem(key);
+    const now = Date.now();
+    if (!seen) {
+      localStorage.setItem(key, String(now));
+      setShowAnnouncement(true);
+      return;
+    }
+    const elapsed = now - Number(seen);
+    setShowAnnouncement(elapsed < 3 * 24 * 60 * 60 * 1000);
+  }, []);
   const userLocalTime = useUserLocalClock();
   const { setThemeMode, themeMode } = useScanTerminalTheme();
   const lastMapSelectedCityRef = useRef<string>("");
@@ -508,6 +522,29 @@ function ScanTerminalScreen() {
             locale={locale}
           />
 
+          {showAnnouncement ? (
+          <section className="scan-upgrade-announcement" aria-label={isEn ? "Upgrade announcement" : "升级公告"}>
+            <div className="scan-upgrade-announcement-copy">
+              <span>{isEn ? "v1.5.6 upgrade" : "v1.5.6 升级公告"}</span>
+              <strong>
+                {isEn ? "Scan terminal is upgraded to v1.5.6" : "决策终端已升级到 v1.5.6"}
+              </strong>
+              <p>
+                {isEn
+                  ? "City decision cards have been redesigned with a compact hero layout and consistent DEB data source. Sticky headers are removed for smoother scrolling."
+                  : "城市决策卡 hero 布局重新设计，三指标并列对比更直观；DEB 数据源统一不再出现不一致；去除顶部固定效果滚动更流畅。"}
+              </p>
+            </div>
+            <ul>
+              <li>{isEn ? "Redesigned decision card hero" : "重设计城市决策卡 hero 布局"}</li>
+              <li>{isEn ? "Unified DEB data source" : "统一 DEB 数据源"}</li>
+              <li>{isEn ? "Light theme coverage" : "亮色主题补全覆盖"}</li>
+              <li>{isEn ? "HKO observatory AI read" : "香港天文台观测 AI 解读"}</li>
+              <li>{isEn ? "Smoother scrolling experience" : "滚动体验优化"}</li>
+            </ul>
+          </section>
+          ) : null}
+
           <section className="scan-list-section">
             <div className="scan-list-header">
               <div className="scan-list-tabs">