[Copilot]

The Node 16 build failed because package.json requested newer date-fns packages than package-lock.json resolved. The upgrade also required imports compatible with the new package exports.

• Dependencies: Align the lockfile with date-fns@4.1.0 and date-fns-tz@3.2.0.
• Calendar imports: Use the updated timezone API and supported package entry point:

  import { endOfWeek, startOfWeek } from 'date-fns';
  import { fromZonedTime } from 'date-fns-tz';

[Jason Johnston (jasonjoh)]

Fix from #326 but with lockfile updated so npm ci succeeds.