🎨 Palette: Add semantic keyboard shortcuts to game instructions

[EiJackGH]

💡 What: Replaced plain text game control instructions with semantic <kbd> tags and added a new physical keycap CSS style.
🎯 Why: The previous text ("Press Space or ↑ to jump!") was hard to scan quickly against the dynamic blue game background. Wrapping the shortcuts in stylized keycaps improves intuitiveness and discoverability for users.
📸 Before/After: Visual changes verified via Playwright screenshot; the Space and ↑ commands now look like physical buttons.
♿ Accessibility: Using the semantic <kbd> tag is better for screen readers, clearly denoting user input commands.

Note: Custom CSS was added to the internal <style> block because this repository manages styling for mario-game.njk within that specific block rather than through a global utility class system (like Tailwind).

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PR created automatically by Jules for task 16804796162236278959 started by @EiJackGH

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