Merge branch '7-example-connection-to-frontend' into 'main'

backend/src/index.ts

frontend/android/AndroidManifest.xml

frontend/core/src/com/game/tankwars/Callback.java

frontend/core/src/com/game/tankwars/ReceiverHandler.java

...c/com/game/tankwars/controller/LeaderboardController.java

.../src/com/game/tankwars/controller/MainMenuController.java

frontend/core/src/com/game/tankwars/model/User.java

...nd/core/src/com/game/tankwars/view/LeaderboardScreen.java

frontend/core/src/com/game/tankwars/view/MainMenuScreen.java