NetworkStatus indicator does not recover when browser goes back online

[Kingsman-99]

Description

NetworkStatus shows an "offline" banner when the browser loses connectivity, but it does not remove the banner when connectivity is restored. The component should listen to the window online and offline events and update the indicator accordingly.

Acceptance Criteria

• The component subscribes to window.addEventListener('online', ...) and window.addEventListener('offline', ...)
• Event listeners are removed on component unmount
• The banner disappears (or shows "Back online" briefly) when connectivity is restored
• Existing tests continue to pass

Context

• Target file: src/components/NetworkStatus.tsx