🎨 Palette: Highlight keyboard shortcuts

[EiJackGH]

💡 What: Added semantic <kbd> tags and physical-key styling to the text instructions for jumping in the Mario game.
🎯 Why: Users playing web-based games often overlook plain text instructions. Formatting them as distinct physical keys makes the required keyboard interactions immediately obvious and intuitive.
📸 Before/After: The instructions changed from plain text to highlighted, shadow-bordered keycap shapes that stand out clearly against the blue sky background.
♿ Accessibility: Improves semantic markup by explicitly defining keyboard inputs using the native HTML <kbd> element, which is the correct semantic element for user input.

Note on constraints: Custom CSS was appended to the internal <style> block in mario-game.njk because this specific file does not utilize a utility class framework (like Tailwind) and relies entirely on its own internal <style> block for layout and design. Adding the new selector directly to the existing block is the cleanest and most consistent way to integrate the enhancement without refactoring the page.

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PR created automatically by Jules for task 313597686587067868 started by @EiJackGH

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