feat(KB-173): add Zen theme to dashboard

- Add Zen to COLOR_THEMES array in core types
- Add Zen theme CSS styles with dark and light variants
- Add Zen theme option to ThemeSelector component
- Add tests for Zen theme selection in ThemeSelector
- Update index.html theme initialization to include Zen
- Add changeset for the new theme

.changeset/add-zen-theme.md

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+"@dustinbyrne/kb": minor
+---
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+Add new "Zen" color theme - a minimal, monochrome theme with subtle accents and reduced visual noise