prefer client timezone for site announcements

src/web/pages/SiteAnnouncements.tsx

@@ -6,6 +6,7 @@ import { clearFocusParams, readFocusAnnouncementId } from './helpers/navigationF
 import {
   formatSiteAnnouncementSeenAt,
   readClientTimeZone,
+  resolveSiteAnnouncementTimeZone,
   SiteAnnouncementContent,
 } from './helpers/siteAnnouncementPresentation.js';
 import { tr } from '../i18n.js';
@@ -44,7 +45,7 @@ export default function SiteAnnouncements() {
   const rowRefs = useRef<Map<number, HTMLDivElement>>(new Map());
   const highlightTimerRef = useRef<number | null>(null);
   const viewerTimeZone = useMemo(() => readClientTimeZone(), []);
-  const displayTimeZone = serverTimeZone || viewerTimeZone;
+  const displayTimeZone = resolveSiteAnnouncementTimeZone(viewerTimeZone, serverTimeZone);
 
   const siteNameById = useMemo(() => {
     const map = new Map<number, string>();

src/web/pages/helpers/siteAnnouncementPresentation.test.tsx

@@ -5,6 +5,7 @@ import {
   SiteAnnouncementContent,
   formatSiteAnnouncementSeenAt,
   readClientTimeZone,
+  resolveSiteAnnouncementTimeZone,
 } from './siteAnnouncementPresentation.js';
 
 type BrowserGlobals = {
@@ -85,6 +86,11 @@ describe('siteAnnouncementPresentation helpers', () => {
     expect(formatSiteAnnouncementSeenAt('2026-03-20 04:23:27', 'Asia/Shanghai')).toBe('2026/03/20 12:23:27');
   });
 
+  it('prefers client timezone over server timezone', () => {
+    expect(resolveSiteAnnouncementTimeZone('Asia/Shanghai', 'UTC')).toBe('Asia/Shanghai');
+    expect(resolveSiteAnnouncementTimeZone('', 'UTC')).toBe('UTC');
+  });
+
   it('reads the browser timezone when available', () => {
     expect(readClientTimeZone()).toBeTruthy();
   });

src/web/pages/helpers/siteAnnouncementPresentation.tsx

@@ -340,6 +340,17 @@ export function readClientTimeZone(): string | undefined {
   }
 }
 
+export function resolveSiteAnnouncementTimeZone(
+  clientTimeZone?: string | null,
+  serverTimeZone?: string | null,
+): string | undefined {
+  const client = String(clientTimeZone || '').trim();
+  if (client) return client;
+
+  const server = String(serverTimeZone || '').trim();
+  return server || undefined;
+}
+
 export function formatSiteAnnouncementSeenAt(
   value: string | null | undefined,
   timeZone = readClientTimeZone(),

src/web/pages/siteAnnouncements.test.tsx

@@ -21,6 +21,14 @@ vi.mock('../api.js', () => ({
   api: apiMock,
 }));
 
+vi.mock('./helpers/siteAnnouncementPresentation.js', async () => {
+  const actual = await vi.importActual<typeof import('./helpers/siteAnnouncementPresentation.js')>('./helpers/siteAnnouncementPresentation.js');
+  return {
+    ...actual,
+    readClientTimeZone: () => 'Asia/Shanghai',
+  };
+});
+
 async function flushMicrotasks() {
   await act(async () => {
     await Promise.resolve();
@@ -107,7 +115,7 @@ describe('SiteAnnouncements page', () => {
       expect(apiMock.getRuntimeSettings).toHaveBeenCalled();
       expect(JSON.stringify(root!.toJSON())).toContain('站点公告');
       expect(JSON.stringify(root!.toJSON())).toContain('Maintenance');
-      expect(JSON.stringify(root!.toJSON())).toContain('2026/03/20 04:23:27');
+      expect(JSON.stringify(root!.toJSON())).toContain('2026/03/20 12:23:27');
 
       const highlightedRows = root!.root.findAll((node) => {
         const className = typeof node.props.className === 'string' ? node.props.className : '';