feat(labs): add theme controls and service for user theme management

[LibbyUX]

Summary

• feat(labs): add theme preference controls and service for user theme management
  • ThemePreferenceSelector variant: Material single select button toggle
    • Used in spacious drawer menus
  • ThemePreferenceMenu: Material icon button that opens mat-menu
    • Used in the header

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Command	Status	Duration	Result
nx affected -t lint test build build-compodoc b...	❌ Failed	1m 30s	View ↗
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nx-cloud record -- commitlint --from f3ab041784...	✅ Succeeded	<1s	View ↗

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☁️ Nx Cloud last updated this comment at 2026-07-23 17:29:39 UTC

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