#10275: feat: add multiligual support for UI

add multiligual support for UI <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Added UI internationalization scaffolding: locale dictionaries (en/zh), an i18n manager with localStorage + browser-language detection, and a localeChanged event for rerendering. - Updated the UI topbar/nav/content header to use translation keys and added a language selector dropdown. - Added i18n-specific CSS and docs/test artifacts (UI_I18N_GUIDE.md, i18n-test.html, README-zh.md + README link). <h3>Confidence Score: 3/5</h3> - This PR is close to mergeable but has at least one user-visible i18n regression due to mismatched translation keys. - Core i18n wiring looks straightforward, but `app-render.ts` references keys that don't exist in the locale maps, which will surface as raw key strings in the UI (affecting accessibility labels). No other definite runtime-breaking issues were identified in the diff. - ui/src/ui/app-render.ts, ui/src/ui/i18n/locales.ts <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->