feat(add-on): add TanThemer (@lonik/themer)

Which project does this relate to?

Create TanStack App

Describe the feature

There are already several great options for theming, but I’d love to see @lonik/themer added as an official add-on for TanStack Start.

Despite being a relatively small library, @lonik/themer provides a clean and modern approach to theme management that fits particularly well with TanStack Start. In my experience, it is a great alternative to next-themes, especially for applications that want a dedicated TanStack Start-friendly theming solution.

Having it available as an add-on would make it much easier to start a new TanStack application with a properly configured theme system from the beginning.

Additional context

• @lonik/themer on npmx.dev
• next-themes on npmx.dev

Issue では実装ファイルやテストは特定されていません。まず、既存の公式 add-on を TanStack CLI がどのように定義してパッケージ化しているかを確認し、次に @lonik/themer のパッケージドキュメントを確認して生成される setup を特定してください。TanStack Start アプリで add-on を選択し、動作するテーマ設定を受け取れるようになれば完了です。