Feat: demo songs

src/state/ui.rs

@@ -79,6 +79,8 @@ pub struct UiState {
     pub zoom_factor: f32,
     pub window_width: u32,
     pub window_height: u32,
+    pub load_demo_on_startup: bool,
+    pub demo_index: usize,
 }
 
 impl Default for UiState {
@@ -127,6 +129,8 @@ impl Default for UiState {
             zoom_factor: 1.5,
             window_width: 1200,
             window_height: 800,
+            load_demo_on_startup: true,
+            demo_index: 0,
         }
     }
 }