Added a header language switcher and `SITE_DEFAULT_LANG` for root-path language and URL prefixes. | Dev Logs

Date: 2026-01-10 20:36 Summary: Added a header language switcher and SITE_DEFAULT_LANG for root-path language and URL prefixes.

[question] 在页面菜单上增加语言选择功能，并提供默认语言

[try to solve] Rendered a language control in the build-time header:

• When SITE_LANGS has multiple languages, show a dropdown with per-page links for each enabled language.
• When only one language is enabled, still show the current language as a disabled control so a switcher appears once more languages are enabled.

Default language:

• Added SITE_DEFAULT_LANG (default zh at the time) for which language maps to /.
• Default language has no URL prefix; others use /<lang> (e.g. /en).

[actions]

• Updated scripts/site/config.mjs (SITE_DEFAULT_LANG, prefix derivation)
• Updated scripts/site/components.mjs (header language UI)
• Updated README.md (default-language usage)