Use nanonext http server for preview

[shikokuchuo]

Summary

• Serve previewed sites over http://127.0.0.1 (ephemeral port) instead of opening file:// paths directly, enabling dynamic features like the Algolia search bar that require HTTP to function.
• The server persists across preview_site() calls for the same site root and is replaced when switching sites.
• Uses a lightweight C-level static directory server, bypassing R entirely after creation.
• Adds nanonext (>= 1.8.0) to Imports.

Details

• A package-level environment (the) caches the active server and its root path across calls within a session.
• stop_preview() explicitly closes the server. It's exported for manual use but not strictly necessary as the server is automatically replaced when switching sites and closed when the R session ends.
• New tests cover server creation, reuse, replacement on root change, stop_preview() cleanup, and URL construction for sub-paths.

[maelle]

Cool! Another docs section that would need updating: https://pkgdown.r-lib.org/reference/build_search.html#debugging-and-local-testing

[shikokuchuo]

@maelle thanks, updated now.