fix(unplugin): generate theme templates as real files

[benjamincanac]

🔗 Linked issue

Resolves #4951

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Fixes the Vite plugin where theme files were incorrectly shipped inside .nuxt/ on NPM (#build/ui.css import was not working).

They are now properly generated as real files that Tailwind CSS can read, ensuring custom theme colors and user configuration work correctly.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@5228

commit: fc60fc4