feat(ui): add in-UI language switcher with persisted preference

[zhangweildlh]

Summary

Add an in-UI language switcher to the graph UI so users can pick Chinese / English / Follow browser without editing config files or restarting. The choice persists to CBM_CONFIG_UI_LANG and survives reloads.

Changes

• C (src/ui/http_server.c): POST /api/ui-config now persists ui-lang (en/zh/auto) via cbm_config_set; GET /api/ui-config echoes lang_pref (en/zh/auto).
• C tests (tests/test_httpd.c): new ui_server_ui_config_post_persists_lang covers POST persistence and GET echo.
• TS (graph-ui/src/lib/i18n.ts): add UiLangPref, langOptionLabels, getUiLangPref, setUiLanguage (POST + re-render).
• TS (graph-ui/src/App.tsx): render a language <select> in the header.
• TS tests (graph-ui/src/lib/i18n.test.ts): cover setUiLanguage and option labels.

Testing

• Frontend: tsc -b passes; vitest 9/9 pass (incl. new switcher tests).
• C: tests/test_httpd.c extended; compiled and run by upstream CI (this environment has no C toolchain, so CI validates the C side).

Notes

• Non-breaking: default remains auto (follow Accept-Language). Nothing changes for existing users.
• Single-issue, <500 lines, builds on the existing CBM_CONFIG_UI_LANG mechanism (no new config keys).
• Fixes feat(ui): add an in-app language switcher to the graph UI #1668

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.

[DeusData]

Thank you for keeping this on the existing ui-lang setting and for adding both C and frontend regressions. The PR is focused and dependency-free. Review will concentrate on the new local POST boundary, strict value and request validation, atomic persistence, and preserving the HTTP server's host and origin defenses; changing language must not become a reason to widen general MCP-over-HTTP access. The queue is full, but the contribution is labeled and routed for normal feature review.