🎨 Palette: Add explicit empty state for highscore

[EiJackGH]

💡 What: Added an explicit empty state when the user has no highscore.
🎯 Why: To clarify the system state and improve user onboarding instead of hiding the personal best line completely.
📸 Before/After: Before, no personal best was displayed if the highscore was 0. After, it shows "Personal Best: None yet! Go set one!".
♿ Accessibility: Improves cognitive accessibility by providing an explicit and encouraging system status.

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PR created automatically by Jules for task 8851846643600692844 started by @EiJackGH

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