🎨 Palette: Add keyboard shortcut highlighting

[EiJackGH]

💡 What: Added semantic <kbd> tags around the keyboard shortcut instructions ("Space" and "↑") in the Mario game UI, and applied a subtle "physical key" CSS style to them.

🎯 Why: When building interactive HTML5 widgets or games with custom keyboard event bindings, users may not intuitively know which keys to press, or the instructions might blend into the background. Distinctly styling the keys reduces cognitive load and makes the controls immediately obvious.

📸 Before/After:
(Visual change: the text "Press Space or ↑ to jump!" now has "Space" and "↑" styled like physical keyboard buttons with a light background and a subtle shadow for depth.)

♿ Accessibility: Using semantic <kbd> tags programmatically identifies these elements as user input keys, which assistive technologies can utilize. The added text-shadow ensures high contrast against the dynamic game background.

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PR created automatically by Jules for task 5970282989697870651 started by @EiJackGH

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