🎨 Palette: Add keyboard shortcut hints and text contrast

[EiJackGH]

💡 What: Wrapped the "Space" and "↑" instructions in <kbd> tags and applied physical key styling via CSS. Added a text-shadow to both the instructions and the score container.

🎯 Why: To make the keyboard shortcuts more discoverable and intuitive by giving them a visual affordance of being pressable keys. The text shadow ensures the white text remains legible regardless of any dynamic changes to the background.

📸 Before/After:
(See UI verification screenshot in PR comments)

• Before: Plain text "Press Space or ↑ to jump!"
• After: "Space" and "↑" look like physical keyboard buttons, with improved contrast.

♿ Accessibility: Using semantic <kbd> tags structurally identifies these elements as user input for assistive technologies. Text-shadow improves the visual contrast ratio against the blue sky background.

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PR created automatically by Jules for task 5263360877476785077 started by @EiJackGH

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