🎨 Palette: Highlight keyboard shortcuts with <kbd> tags

[EiJackGH]

💡 What: Added <kbd> tags and CSS styling to make keyboard shortcut instructions look like physical keys.
🎯 Why: To make the instructions more intuitive, visually distinct from plain text, and immediately recognizable as keyboard inputs.
📸 Before/After: The plain text "Space or ↑" is now rendered as distinct, styled keycaps.
♿ Accessibility: Visually separates instructional text from action inputs, improving readability.
Note: Added custom CSS specifically because the mario-game.njk file relies entirely on a single internal <style> block rather than utility classes.

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PR created automatically by Jules for task 3229901429414332548 started by @EiJackGH

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