🎨 Palette: Improve Welcome page accessibility and scannability

[christopherfoxjr]

This PR implements several micro-UX improvements focused on accessibility and scannability for KibaOS:

1. Welcome Page Accessibility:
   • Uses semantic <main> tag for primary content.
   • Implements ARIA list roles for better screen reader support in feature and design sections.
   • Adds high-contrast focus indicators (:focus-visible) for keyboard navigation.
2. Visual Scannability:
   • Adds intuitive emojis (📖, 🐛, 🐙) to buttons to help users quickly identify actions.
3. Installer Accessibility:
   • Adds ARIA-equivalent properties to the KibaOS branding logo in the Calamares installer slideshow.
4. Hygiene:
   • Removes one trailing whitespace in build.sh to satisfy repository audit requirements.

Verified using Playwright screenshots and grep. No temporary artifacts or redundant build infrastructure included.

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PR created automatically by Jules for task 1062747916141388478 started by @christopherfoxjr

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