Configure Buildozer presplash for launcher-time app loading

[Copilot]

This issue asks for app loading to be set up in the Buildozer launcher flow. The change wires an explicit presplash asset so Android shows a loading screen before the Pygame app is ready.

• Launcher loading configuration

  • Added Android presplash configuration in buildozer.spec:
    • presplash.filename = bg.png
  • This enables a defined splash screen during startup from the launcher path.

• Packaging compatibility

  • Reused existing root asset (bg.png) and current include settings (include_exts already includes png), so no additional asset pipeline changes were needed.

# buildozer.spec
include_exts = py,png,jpg,jpeg,ttf,txt
presplash.filename = bg.png