🎨 Palette: Keyboard Shortcut Discoverability

[EiJackGH]

💡 What: Added semantic <kbd> tags and physical key styling to the instructions, and applied a text-shadow for better contrast against the dynamic blue background. Appended custom CSS to the internal <style> block.
🎯 Why: Improves intuitiveness and discoverability of custom keyboard controls in the Mario game widget.
📸 Before/After: Before, the instructions ("Press Space or ↑ to jump!") were plain text and hard to read against the blue background. After, the keys are styled like physical buttons and the text is more legible.
♿ Accessibility: Semantic <kbd> tags improve screen reader context for shortcuts. Custom CSS was appended to the internal <style> block to match the existing pattern in the file, as there is no external stylesheet.

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PR created automatically by Jules for task 4660396323191911955 started by @EiJackGH

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