feat(frontend): add bilingual app foundation and public content browsing

[Typogalaxy]

Summary

Establishes the initial application foundation for the Nuxt frontend and adds public browsing for community posts and published articles.

What changed

• Add the authentication foundation, including session state, login flow, route middleware, and user menu
• Add reusable UI primitives for buttons, inputs, form fields, errors, loading states, empty states, and avatars
• Add English and Chinese localization for the application shell and new pages
• Add public, SSR-compatible community post list and detail pages
• Add public, read-only article list and detail pages
• Add pagination and loading, empty, error, and not-found states
• Show the latest articles and community posts on the homepage
• Allow anonymous users to read community posts, published articles, and comments
• Sanitize rendered article HTML before returning it to clients
• Add focused frontend and backend tests for the new API and browsing behavior
• Document the frontend internationalization architecture in English and Chinese

Notes

• Article publications remain read-only through the public API
• Unpublished articles are not exposed
• The frontend uses the generated OpenAPI types through the existing API layer
• Content pages support server-side data fetching for initial rendering

Closes #67