Clear now-playing.txt when the app closes

[paulpv]

Problem

• The player writes the current track name to now-playing.txt, but the file still contains stale data after the app is closed.
• Downstream tools that read now-playing.txt (e.g., overlays or stream widgets) continue to show the last track even though nothing is playing.

Proposal

• Detect when the app window or tab is being closed and clear the contents of now-playing.txt before exiting.
• Consider also clearing the file if playback stops because the selected folder is closed or the track list empties.

Acceptance Criteria

• Closing the player tab or window results in an empty now-playing.txt file.
• Clear behavior does not trigger while general playback is paused or when switching tracks.
• Behavior works consistently across supported Chromium-based browsers.

Notes

• Ensure the cleanup logic aligns with the File System Access API limitations and handles permission prompts gracefully.