fix(i18n): make a chosen language follow the reader off the static pages - #197

Reported: pick a language on the homepage, go anywhere else, and it is
English again. Reproduced exactly -- /ja/ is Japanese, and the moment you
click "保存したドキュメント" you are on /history in English.

The site is seven directories of static pages plus one app (/editor and
/history) that lives under none of them. So there were two separate
failures, and both had to go:

1. Links into the app did not carry the language. The generated homepage
   linked to a bare /history; history-recent.js built bare
   /editor?saved=... links; /history itself sent people back to "/" and
   opened documents without a locale. All of them now carry it, through
   two small helpers exported from the shell's i18n (localeHomePath,
   withLocale) so the app and the generator agree on where a language's
   homepage is.

2. The choice was never remembered. lang-switch.js only navigated: the
   language existed as the path the reader happened to be standing on and
   nowhere else. It now writes a `locale` cookie before navigating, which
   is the second entry in the app's resolution chain (?locale=, cookie,
   localStorage, browser).

Both are needed. The parameter makes the first click right -- a fresh
browser, a shared link, someone who never touched the switch. The cookie
makes everything after it right, including typing /editor directly.

Walked the whole path in a browser reporting en-US: English homepage ->
pick 日本語 -> saved documents -> bare /editor -> bare /history ->
a satellite page. Six steps, all lang="ja". Reverse-checked: remove the
cookie and steps four and five fall back to English.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>