Upgrade Astryx and add selectable themes

[walishcs]

What changed

• Upgrade Astryx Core and CLI from 0.2.0 to 0.3.0.
• Add pinned Stone, Neutral, Matcha, and Chocolate theme packages at 0.3.0.
• Add a Keystatic Site theme selector with Stone as the safe default.
• Add a centralized, extensible theme registry and load precompiled CSS for all supported themes.
• Preserve system, light, and dark modes across the selected site theme.
• Update the repository workflow so template-only content cleanup never reaches main.
• Add unit coverage for supported themes and fallback behavior.

Content safety

This integration branch was created from current main and contains no src/content or public asset changes. Existing site settings and public content remain unchanged. The personal site settings removal stays only on template.

Validation

• npm run check
• npm test (30 tests)
• npm run format:check
• npm run build (18 Pagefind pages indexed)
• npm run astryx -- doctor
• Verified the template also builds successfully with site-settings.yaml absent.