Add colourblind mode setting

[JavadTe]

Summary

This pull request adds a simple colourblind mode setting to improve accessibility.

Changes

• Added a new colourblind_mode setting.
• Added a Colourblind mode option to the Settings menu.
• Added high-contrast snake colours when colourblind mode is enabled.
• Added high-contrast board colours when colourblind mode is enabled.

Testing

• Ran the game locally using py -m uv run naja.
• Confirmed the Colourblind mode option appears in Settings.
• Confirmed the game colours change when Colourblind mode is On.
• Ran py -m uv run ruff check . and all checks passed.
• Ran py -m uv run pytest. Result: 349 passed, 1 failed. The failed test is related to game over high score behaviour and appears unrelated to this colourblind mode change.

Related issue

Closes #511