🎨 Palette: Add accessibility and instructions to Mario game UI

[EiJackGH]

💡 What: Added aria-live attributes to the #score element and added explicit on-screen instructions for game controls in src/views/mario-game.njk.
🎯 Why: To ensure screen readers announce score updates dynamically without needing to re-focus, and to make it immediately obvious to users which keys are required to play the game without guesswork.
📸 Before/After: Before, the score lacked accessibility attributes and there was no visual indication of controls. Now, the score is screen-reader friendly and "Press Space or Up Arrow to jump" is visible.
♿ Accessibility: Added aria-live="polite" and aria-atomic="true" to #score so changes are announced non-intrusively. Kept static instructions out of the dynamic region to avoid redundant repetition.

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PR created automatically by Jules for task 13771026083174511345 started by @EiJackGH

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