🎨 Palette: [UX improvement] Highlight keyboard shortcuts

[EiJackGH]

💡 What: Added semantic <kbd> tags and physical key styling to the game's keyboard instructions (Space and ↑) and a text-shadow.
🎯 Why: Enhances discoverability and makes the interface more intuitive by visually mapping the text to physical keys, ensuring text is readable against dynamic backgrounds.
📸 Before/After: Screenshots generated during verification show plain text "Press Space or ↑ to jump!" replaced with explicitly styled keycaps.
♿ Accessibility: Improves semantics for screen readers by highlighting keyboard interactions. Custom styling appended to the existing internal <style> block is necessary to match the file's current pattern.

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PR created automatically by Jules for task 11412166371519405095 started by @EiJackGH

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