#2294 web: an optional interface language (zh-CN first), English unchanged

Description

A small layer (serve/web/i18n.js) walks the DOM and replaces the interface's English text with the chosen language. Chat messages, reasoning, tool output, code blocks and form fields are never touched, and a MutationObserver handles text the app adds later (toasts, status pills, monitor rows).

- English strings are the keys, so nothing changes for English users.
- The choice is remembered (localStorage "strata.lang"); without it the browser's own language decides.
- A header button switches back and forth; switching to English restores the original text (no reload), so it cannot get stuck.
- tokens.css adds CJK fallbacks to the font stack.
- app.js/monitor.js: five interface strings that land in skipped areas go through tr().

## Title
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Issue: Resolves # 

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