feat(scene): implement skeleton main menu

Doesn't have anything implemented inside yet, but we'll try.

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

@@ -0,0 +1,19 @@
+package org.vibecoders.moongazer.scene;
+
+import static org.vibecoders.moongazer.Constants.*;
+
+import com.badlogic.gdx.graphics.g2d.SpriteBatch;
+
+/**
+ * Main menu scene.
+ */
+public class MainMenu extends Scene {
+    /**
+     * Renders the main menu scene.
+     * @param batch The SpriteBatch to draw with.
+     */
+    @Override
+    public void render(SpriteBatch batch) {
+        batch.draw(TEXTURE_WHITE, 0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
+    }
+}