Improve offline caching, stale data handling, and sync resilience for user state

[grantfox-oss]

Make cached frontend data safer by adding stale-while-revalidate patterns, explicit sync fallbacks, and clearer user status when data may be outdated.

• Affected files: app/frontend/src/lib/watchlist.ts, app/frontend/src/components/NotificationCenterProvider.tsx, app/frontend/src/app/notifications/page.tsx, app/frontend/public/sw.js, app/frontend/src/app/layout.tsx, app/frontend/src/components/PWAHandler.tsx, app/frontend/src/app/offline/page.tsx, app/frontend/src/lib/api.ts, app/frontend/src/app/dashboard/page.tsx
• Acceptance criteria:
  • Cached user data clearly distinguishes fresh, stale, and unavailable states.
  • Background sync or refresh behavior does not overwrite newer local state with stale responses.
  • Offline-and-refresh transitions are documented and validated for critical client data sets.

[felladaniel36-hash]

I'd like to claim this issue. I have reviewed the details and can begin working on a fix right away. Please assign it to me if it's still available. Thanks!

[Joyful12-tech]

Hello Maintainer! I'd like to work on this issue. I can deliver it following the requirements and submit a quality PR within 2 days. Thank you!

[grantfox-oss]

🦊 GrantFox — @Joyful12-tech has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #534)
2. Your PR will be reviewed by the BlockDash-Studios maintainers

Good luck! Track your progress on GrantFox.