🎨 Palette: Add keyboard instructions and improve score accessibility

[EiJackGH]

💡 What:
Added visible instructional text ("Press Space or Up Arrow to jump") for the keyboard controls and improved screen reader accessibility for the score display.

🎯 Why:
Users (especially non-gamers) might not intuitively know which keys to press. Furthermore, the dynamic score text was not properly announced by screen readers, and replacing the entire "Score: X" string repeatedly is bad practice for a11y.

📸 Before/After:
Before: No visible instructions, score updated entire text block.
After: Explicit instructions in top right corner, score updates only the number within an aria-live span.

♿ Accessibility:

• Added aria-live="polite" and aria-atomic="true" to the dynamic score <span id="score-value"> so screen readers only announce the updated score number without endlessly repeating the static "Score: " text.
• Ensured keyboard instructions have sufficient color contrast with a semi-transparent dark background.

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PR created automatically by Jules for task 9196131454668226907 started by @EiJackGH

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