fix(web): persist locale cookie on canonical docs pages

[zerone0x]

Issue for this PR

Closes #15845

Type of change

• Bug fix

What does this PR do?

The language switcher on opencode.ai/docs doesn't stick when switching to English (or any locale via the Starlight switcher).

Root cause: the oc_locale cookie was only written in one place — inside docsAlias(), which triggers on alias URLs like /docs/en/page and redirects to the canonical path. The Starlight language switcher generates canonical URLs directly (e.g. /docs/page for English, /docs/tr/page for Turkish), so the cookie was never written when switching language this way. On the next visit to /docs/, the middleware re-read Accept-Language from the browser and redirected back to the original locale.

The fix detects the locale from the URL path on every docs page request and appends the oc_locale cookie to the response when the current cookie value differs. This makes the language switcher choice sticky without requiring any Starlight or build-time changes.

How did you verify your code works?

Traced the middleware logic manually:

• /docs/tr/page → sets oc_locale=tr if cookie differs
• /docs/page → infers root/English (no locale prefix → exactLocale returns null → "root") → sets oc_locale=en
• Subsequent /docs/ → reads cookie → no redirect ✓
• Already-matching cookie → no Set-Cookie header (no overhead per request) ✓

Screenshots / recordings

N/A — middleware logic change, no visual output.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential duplicate found:

• PR fix(web): persist English on unprefixed docs routes #13756: "fix(web): persist English on unprefixed docs routes"
  fix(web): persist English on unprefixed docs routes #13756

Why it might be related:
PR #13756 appears to address a similar issue with persisting locale on docs routes. Both PRs deal with fixing language/locale persistence on documentation pages. You should check if #13756 was already merged or if it addressed the same underlying problem differently.