[amamsy90]

Summary

• Fixes Missing end-of-game experience #132. After the snake hit a wall or itself, the window froze on a red square and never said the game was over.
• Draw a centered red Game Over message, then stop the loop the same way as before.

Test plan

• Reproduced the bug with the existing mocked turtle run (random.seed(0) plus the arrow-key sequence in tests/test_snake.py). The snake grew (Snake: 2) and no text was drawn.
• After the change, pytest tests/test_snake.py passes and snake.py stays at 100% coverage. The test asserts that Game Over is written.
• pytest tests: 45 passed.
• Play python -m freegames.snake, crash into a wall, and confirm the message appears in the window.