Frontend: Add offline support for puzzles via Service Workers

[chinweobtagaz]

Description

Users on unstable connections (e.g., commuting) should be able to train using chess puzzles even when disconnected.

What Needs to be Done

• Update
  ext.config.js to implement next-pwa.
• Configure service workers to cache a payload of 100 daily puzzles.
• Create an 'Offline Mode' banner when
  avigator.onLine is false.
• Store completed puzzle results in IndexedDB and sync them to the backend when reconnected.

Acceptance Criteria

• App is installable as a PWA.
• When offline, users can still access the 'Daily Puzzles' section.
• Solved puzzles are queued locally and synced perfectly upon network restoration.
• No blank screens or unhandled network errors when navigating offline.