prefer client timezone for site announcements

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(),