chore(scene): remove abstract from dispose

Not all scenes need to have the custom dispose method.

app/src/main/java/org/vibecoders/moongazer/scene/Scene.java

@@ -4,5 +4,7 @@ import com.badlogic.gdx.graphics.g2d.SpriteBatch;
 
 public abstract class Scene {
     public abstract void render(SpriteBatch batch);
-    public abstract void dispose();
+    public void dispose() {
+        // Default implementation does nothing
+    }
 }