🎨 Palette: Add semantic <kbd> tags for keyboard shortcut discoverability

[EiJackGH]

💡 What: Wrapped the keyboard shortcut instructions ("Space" and "↑") in the Mario Game with semantic <kbd> tags and applied physical key styling (border, background, box-shadow) via CSS. Also added a text shadow to the instruction container to ensure the text remains legible over the game's background.

🎯 Why: To make the interface more intuitive and the controls more discoverable. Highlighting keyboard shortcuts visually reduces cognitive load for users and provides clear, immediate affordance for interactive game controls.

📸 Before/After: Before, the text "Space or ↑" blended in with the rest of the instructional sentence. After, "Space" and "↑" look like physical keys on a keyboard, clearly indicating they are user inputs. (Validated via Playwright screenshot).

♿ Accessibility: Using semantic <kbd> tags is the appropriate HTML standard for representing user input. It provides better structure for assistive technologies and improves the overall semantic meaning of the document.

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PR created automatically by Jules for task 3226167368258930428 started by @EiJackGH

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