Remove hardcoded built-in music tracks

Simplify MusicManager to use only custom audio files:

Changes:
1. MusicManager Constructor:
   - Initialize tracks array as empty instead of with 4 hardcoded tracks
   - Custom tracks loaded immediately by loadCustomTracks()
   - No more built-in vs custom track distinction needed

2. refreshCustomTracks() Method:
   - Simply clear tracks array since no built-in tracks exist
   - Reload custom tracks without filtering logic
   - Cleaner implementation without isBuiltIn property logic

Benefits:
 Music player now shows only user's imported background music
 Simpler codebase without hardcoded audio references
 All tracks are treated equally as custom content
 No need to maintain separate built-in/custom track categories

The music player will now be populated entirely from the user's
imported background audio files through the audio management system.

game.js

@@ -3182,12 +3182,8 @@ class MusicManager {
         this.currentTrackIndex = this.dataManager.getSetting('music.currentTrack') || 0;
         this.playHistory = [];
         
-        this.tracks = [
-            { name: 'Colorful Flowers', file: 'audio/Colorful-Flowers(chosic.com).mp3', isBuiltIn: true },
-            { name: 'New Beginnings', file: 'audio/New-Beginnings-chosic.com_.mp3', isBuiltIn: true },
-            { name: 'Storm Clouds', file: 'audio/storm-clouds-purpple-cat(chosic.com).mp3', isBuiltIn: true },
-            { name: 'Brunch For Two', file: 'audio/Tokyo-Music-Walker-Brunch-For-Two-chosic.com_.mp3', isBuiltIn: true }
-        ];
+        // Initialize empty tracks array - custom tracks will be loaded next
+        this.tracks = [];
         
         // Load and add custom background music
         this.loadCustomTracks();
@@ -3224,8 +3220,8 @@ class MusicManager {
     }
     
     refreshCustomTracks() {
-        // Remove existing custom tracks
-        this.tracks = this.tracks.filter(track => track.isBuiltIn);
+        // Clear all tracks since we only have custom tracks now
+        this.tracks = [];
         
         // Reload custom tracks
         this.loadCustomTracks();