feat: add custom background rendering and speed scaling features for snake gameplay

include/game_state.h

@@ -18,6 +18,7 @@ struct GameState {
     volatile uint16_t snakelength = 0;
     volatile uint16_t snakeDirection = 1;
     volatile uint16_t score = 0;
+    volatile uint16_t applesEaten = 0;
 
     bool gameOver = false;
     bool paused = false;