Wire MusicPlayerService, removing duplicate inline music state from MainWindow

[mock1ngbb]

STATE MIGRATION — move _audioFileReader, _musicProcess, _launcherMusicPath fields from MainWindow into MusicPlayerService.

Extract from MainWindow.axaml.cs:

• LauncherMusicPath property (lines 88-120)
• _audioFileReader field (line 82)
• _musicProcess field (line 84)
• PlayLauncherMusic() method
• StopLauncherMusic() method
• Volume setter (lines 113-120)
• SelectLauncherMusic_Click/ClearLauncherMusic_Click event handlers

New file: Services/MusicPlayerService.cs

• Must implement INotifyPropertyChanged for LauncherMusicPath
• Keep #if WINDOWS conditional compilation for NAudio vs afplay

Verification: dotnet build passes, music playback works on macOS.
Doable by: sonnet/cline-agent under 45 min.