Music player in the sound settings: pick any calm track or automatic

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/settings.rs

@@ -137,6 +137,7 @@ pub struct Settings {
     pub music_volume: f32,
     pub muted: bool,
     pub voice: bool, // Crookfinger's babbling voice
+    pub music_track: String, // music player: "" = automatic, else the id of a calm track that replaces the calm music
     // camera
     pub rotate_speed: f32,
     pub pan_speed: f32,
@@ -177,6 +178,7 @@ impl Default for Settings {
             music_volume: 0.6,
             muted: false,
             voice: true,
+            music_track: String::new(),
             rotate_speed: 1.0,
             pan_speed: 1.25,
             zoom_speed: 1.0,