🎨 Palette: Add explicit empty state for personal best

[EiJackGH]

What:
Modified the src/main.cpp file to include an explicit else block when checking if highscore > 0. If the highscore is 0, the game now displays: Personal Best: None yet. Set a record!. Also appended a corresponding UX learning to .Jules/palette.md.

Why:
Hiding UI elements (like a high score) when they are empty creates an ambiguous state for new users, leaving them unaware that such a feature exists. An explicit, encouraging empty state clarifies the system state, introduces the feature, and improves user onboarding.

Before/After:

• Before: If highscore is 0, no Personal Best section is displayed.
• After: If highscore is 0, the game displays Personal Best: None yet. Set a record!.

Accessibility:
Clarifies system capabilities and expectations for new users instead of relying on discovery through repeated play, which aligns with cognitive accessibility principles regarding clear instructions and predictable states.

---

PR created automatically by Jules for task 6443492757932510627 started by @EiJackGH

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.