🎨 Palette: Semantic Keyboard Shortcuts with <kbd>

[EiJackGH]

💡 What: Wrapped 'Space' and '↑' text in semantic <kbd> tags with physical styling and added a text-shadow to overlaid game text (score and instructions).
🎯 Why: Explicitly highlighting keyboard shortcuts with physical key styling improves intuitiveness and discoverability for users playing the game. High contrast text-shadows improve readability against the dynamic game background.
📸 Before/After: (Verified via Playwright screenshot: text is now clearly readable with shadows and the key instructions stand out as distinct buttons).
♿ Accessibility: Added semantic <kbd> tags. Custom CSS was required because no existing design system token mapped to <kbd> styling in this standalone HTML file context, but it matches the file's pattern of using an internal <style> block.

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PR created automatically by Jules for task 5146839160420909846 started by @EiJackGH

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