🎨 Palette: Add keyboard shortcut discoverability

[EiJackGH]

💡 What: Wrapped the keyboard shortcuts ("Space" and "↑") in semantic <kbd> tags and applied physical key CSS styling (with contrast-enhancing text-shadows) to the internal <style> block. Also added a journal entry documenting this learning in .Jules/palette.md.

🎯 Why: Custom game controls require explicit discoverability. Highlighting them visually reduces cognitive load and ensures they stand out clearly against the dynamic #5c94fc background, making the UI more intuitive.

📸 Before/After:

• Before: "Press Space or ↑ to jump!" (plain white text).
• After: Keys are beautifully styled with borders and shadows resembling physical keys, drawing the user's attention.

♿ Accessibility: Utilizing semantic HTML tags <kbd> signals to assistive technologies that these are distinct keyboard inputs. The applied text-shadow ensures they remain high-contrast regardless of the background rendering behind them.

Note: As mario-game.njk utilizes an internal <style> block instead of an external design system framework or utility classes, standard CSS was appended there directly in keeping with the file's existing structure.

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PR created automatically by Jules for task 4720228069308781581 started by @EiJackGH

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