Add offline indicator

[cdxker]

Summary

Display a visual indicator when the user is offline so they know their connection status.

Details

• Show an indicator (banner, icon, or toast) when the browser detects no network connection
• Use the navigator.onLine API and online/offline events
• Consider showing which cached content is available while offline
• The indicator should be non-intrusive but visible

Acceptance Criteria

• Offline indicator appears when connection is lost
• Indicator disappears when connection is restored
• Styled consistently with the app's dark theme