fix(nuxt): inline global css with ssr: false route rule

[danielroe]

🔗 Linked issue

resolves #21748
resolves #21760

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ 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

With #21573, we now no longer emit global CSS in an entry.css file but instead inline it based on usage. This won't work with SPA mode by default as there is no usage tracking (even though we do explicitly add the entry file id to a modules set if it exists).

This fix ensures we have a modules set on ssrContext in which to inject the 'entry' module that is our source for inlining CSS.

📝 Checklist

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