feat: add fullscreen toggle functionality to NavLayout component

[FelizCoder]

closes #51

[Copilot]

Pull Request Overview

Adds a fullscreen toggle button to the NavLayout component, enabling users to enter and exit browser fullscreen mode.

• Imported fullscreen icons and added isFullscreen state with toggleFullscreen handler
• Updated useEffect to listen for fullscreenchange events and clean up
• Adjusted header layout to include both collapse and fullscreen buttons

Comments suppressed due to low confidence (1)

app/ui/navigation/NavLayout.tsx:26

• The new fullscreen toggle logic isn’t covered by existing tests. Consider adding unit or integration tests that mock document.fullscreenElement and verify that isFullscreen state updates correctly when toggleFullscreen and the fullscreenchange event fire.

  const toggleFullscreen = () => {