feat: implement global browser offline detection and notification banner (#37)

[MayurKharat0390]

Summary

This PR addresses issue #37 by implementing global browser offline detection, a glassmorphic connection toast banner, and offline fallback adaptations on the Contributors page.

Key Changes

• Connection Lifecycle Hook (useOnlineStatus):
  • Subscribes to window connection updates (online, offline) and returns state indicators dynamically.
• Global Network Banner (NetworkStatusBanner):
  • Slides in from the bottom center when the network is lost (amber warning text, blur background).
  • Switches to an emerald green success banner on reconnection, fading out automatically after 3 seconds.
  • Mounted in App.jsx for global route persistence.
• Offline Fetch Bypass (Contributors):
  • Skips network fetches when offline, preventing redundant developer console network error logs.
  • Automatically serves cached contributor lists.
  • Mounts a dismissible alert card notifying the user they are viewing cached data.

Verification & Testing

• Vite Build: Ran npm run build to verify the production build compiles.
• Offline Mocking: Tested toggling offline status inside browser inspection tools; verified the banner slides in/out correctly and the Contributors list gracefully defaults to cache without network requests.

[abhro05]

@MayurKharat0390 Please resolve the conflicts and tag me!