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]

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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.

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