feat(ui): add an in-app language switcher to the graph UI

[zhangweildlh]

Summary

The graph UI already ships complete, parallel English and Simplified-Chinese message catalogs in graph-ui/src/lib/i18n.ts (messages.en / messages.zh), and the backend already computes and exposes a lang field from GET /api/ui-config — derived from the ui-lang config key (CBM_CONFIG_UI_LANG, default auto) or the request's Accept-Language (detect_ui_lang).

However, there is currently no in-app control to choose the language. A user who wants Chinese today must either set CBM_CONFIG_UI_LANG=zh out-of-band (CLI / config store) or change their browser language. The UI itself offers no selector.

Proposal

Add a small, non-breaking language switcher to the UI header with three options:

• 中文 (zh)
• English (en)
• 跟随浏览器 (auto)

Behavior:

• The selection is persisted by a new POST /api/ui-config endpoint that writes CBM_CONFIG_UI_LANG (en / zh / auto) via the existing cbm_config_set path already opened by handle_ui_config (src/ui/http_server.c).
• GET /api/ui-config is unchanged (still returns lang + upstream_issues_url).
• The frontend i18n.ts gains a setUiLanguage() that POSTs the choice, then updates cachedLanguage and notifies listeners so the whole UI re-renders immediately.
• Default remains auto — no behavior change for existing users.

Scope / constraints

• Pure C for the binary change; React/TS for the frontend.
• Adds tests in tests/test_httpd.c (POST path) and graph-ui/src/lib/i18n.test.ts.
• No new dependencies, no new network egress, no hardcoded external URLs (the issues URL stays backend-served per the existing UI security audit).
• Kept under the 500-line / single-issue guidance.

Ask

I'd like to contribute this as a PR. Posting here first per CONTRIBUTING ("open an issue first"). Happy to adjust the endpoint shape or selector placement to your preference before implementing.