🎨 Palette: Add explicit keyboard instructions to Mario Game

[EiJackGH]

💡 What: Added a visual controls hint to the Mario Game UI.
🎯 Why: To make the keyboard interactions ("Space" or "Up Arrow" to jump) discoverable and intuitive for new players without blocking mouse interactions.
♿ Accessibility: Added explicit visual instructions for keyboard controls.

• Tests run successfully via pytest and flake8.
• UI visually verified using Playwright.
• Logged UX learning in .Jules/palette.md.

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PR created automatically by Jules for task 4894330195078927230 started by @EiJackGH

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