[GSoC] LateNightQML: Menu Bar

[xARSENICx]

This PR implements the platform-aware application menu for LateNightQML, using the shared QML menu/action infrastructure while matching the existing LateNight toolbar and both the Classic and PaleMoon color schemes.

Testing the Experimental Skin

Since this is an early experimental milestone, you must first run Mixxx with the developer flag:

./build/mixxx --developer

Once Mixxx is open, switch to the experimental skin:
Preferences -> Interface -> LateNight QML (Experimental)

You can dynamically toggle between the Classic and PaleMoon color schemes under preferences.

New Styling

On Windows and Linux, the application menu is exposed through a hierarchical hamburger menu integrated into the LateNightQML toolbar. Its popup, menu rows, hover states, spacing, and typography use the same visual language as the existing toolbar dropdowns.

On macOS, the same actions are presented through the native system menu bar and the hamburger button remains hidden.

Scope of Changes

Implemented in this PR:

• Platform-aware menu presentation: Uses the native QML menu bar on macOS and the LateNightQML hamburger menu on Windows and Linux.
• Hierarchical hamburger interaction: Opens one submenu at a time on hover, keeps a clicked submenu open, and closes the complete menu when clicking outside.
• LateNight theme integration: Styles the hamburger menu for Classic and PaleMoon using the established toolbar dropdown appearance.
• Unified preferences entry: Uses Preferences as the single settings destination instead of exposing a separate skin-settings menu.

Dependency

Depends on #16849.

Tracking

GSoC: LateNightQML PR-7