🎨 Palette: Add explicit empty state for highscore

[EiJackGH]

💡 What: Modified src/main.cpp to display an explicit empty state ("0 (Play to set a record!)") when the highscore is zero, instead of hiding the personal best section entirely.
🎯 Why: Hiding UI elements like a highscore when empty removes context for new users. Explicit, encouraging empty states clarify system state and improve user onboarding.
📸 Before/After: Before, the highscore line was simply absent if it was zero. Now, it displays an encouraging message.
♿ Accessibility: Provides clearer context to screen readers and all users about the expected data and how to interact with the system to achieve it.

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PR created automatically by Jules for task 8624502526168793830 started by @EiJackGH

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