Add automatic system tray support to FluxTuner GTK

[pitill0]

Summary

Closes #216.

Adds Linux system tray support to the FluxTuner GTK frontend using
StatusNotifierItem and DBusMenu directly through Gio/D-Bus.

The implementation is isolated from the core playback architecture and does
not introduce an additional runtime dependency.

Behavior

When a compatible Linux StatusNotifierWatcher is available:

• the FluxTuner tray icon is registered automatically;
• closing the GTK window hides it instead of terminating FluxTuner;
• playback continues while the window is hidden;
• activating the tray icon restores and focuses the window;
• the tray menu shows the currently playing station;
• Stop stops playback;
• Quit explicitly terminates FluxTuner.

If tray registration is unavailable, FluxTuner gracefully keeps the existing
close-window behavior.

Tray icon

The tray uses the FluxTuner application icon through SNI IconPixmap, with
the standard audio-radio icon retained as a fallback.

The application icon is now included as packaged GTK data so it also works
when running directly from the source tree.

Lifecycle

The tray uses GApplication.hold() / release() rather than hidden dummy
windows.

The Linux SNI backend uses its own D-Bus connection so that destroying the
tray notifier cleanly removes it from the desktop without interfering with
GTK's shared application bus.

Scope

This initial implementation is Linux-only.

macOS keeps its existing GTK lifecycle. Native macOS menu-bar support can be
implemented separately later.

The initial menu intentionally remains small:

• current station
• Show FluxTuner
• Stop
• Quit

Play/pause, volume, favorites and other controls remain possible future
enhancements.

Testing

• Added StatusNotifierItem / DBusMenu protocol tests.
• Added tray menu and playback-state coverage.
• Added packaged icon coverage.
• Added close-to-tray lifecycle regression coverage.
• Manually validated on niri with DankMaterialShell:
  • automatic tray registration;
  • custom FluxTuner icon;
  • window hide/restore;
  • playback continuing while hidden;
  • Stop;
  • Quit;
  • correct tray removal/re-registration.

make gate passes cleanly.