🎨 Palette: Improve Welcome Screen accessibility and scannability

[christopherfoxjr]

This PR enhances the KibaOS Welcome Screen by adding semantic structure, improving keyboard navigation visibility, and adding descriptive visual cues.

💡 What:

• Added <main> tag to welcome.html.
• Added :focus-visible CSS rules for buttons.
• Added emojis and aria-label to "Wiki", "Report Issue", and "GitHub" buttons.
• Fixed trailing whitespace in build.sh.

🎯 Why:

• Improves accessibility for screen reader and keyboard users.
• Increases visual scannability for all users.
• Ensures the "first-run" experience feels polished and modern.

♿ Accessibility:
-landmark navigation via <main>.
-Visible focus indicators.
-Explicit "opens in new tab" announcements for screen readers via aria-label.

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PR created automatically by Jules for task 7441790715551855146 started by @christopherfoxjr

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