🎨 Palette: Add empty state for progression highscore

[EiJackGH]

💡 What: Added an explicit empty state string to the Personal Best display when the high score is zero, replacing the previous behavior of completely hiding the UI element. Also recorded this as a learning in .Jules/palette.md.
🎯 Why: In CLI games with progression, empty states (e.g., 'None yet! Play to set one!') are more encouraging and intuitive for first-time users than omitting data. It explicitly tells the user what the missing data represents and how to achieve it.
♿ Accessibility: Provides immediate visual clarity to new users about the state of the system rather than leaving them guessing about what UI elements are present.

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PR created automatically by Jules for task 7226088526253553195 started by @EiJackGH

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