feat(ui): add always-on-top window toggle and setting for fullscreen

[kopeng-mi]

Summary

• add titlebar button and shortcut to toggle always on top window pinning
• highlight the titlebar toggle in solid white when active, dimming when unpinned
• hide the pin toggle button from the titlebar in fullscreen view to match existing controls
• add settings option in general window settings to pin automatically on fullscreen now playing and unpin on exit

Testing

Tested on Windows 11:

• clicking the titlebar icon pins Sonora above other applications and lights the icon white
• clicking again unpins Sonora and returns the icon to dim gray
• shortcut (Alt+P / Ctrl+Shift+P) toggles pin state
• entering fullscreen with "Pin on now playing" enabled pins the window, and exiting unpins it

Not tested on Linux or macOS. Needs testing and verification on those platforms.