fix: localize page metadata

[Laaaaaaaam]

Summary

• Update the document title and meta description from the active locale.
• Keep the document language attribute synchronized with the locale.

Fixes #786.

Testing

• npm run build
• Verified in the local Vite app that switching between Chinese and English updates the title, description, and lang; the English values persist after reload.

[R-zin]

It localizes the page metadata that you’re explicitly updating.

When the locale changes, it updates:

• Page title → meta.title
• Meta description → meta.description
• HTML language attribute →