feat(web): persist language preference

[seekskyworld]

Problem

Implements the language-preference slice of #350. The Web workbench selected a locale only from the browser and lost the choice on reload.

Value

Users can keep a deliberate English/Chinese preference across reloads without involving Session or credential storage.

Approach

Read a validated openpi.language value from browser-local storage, fall back to navigator detection, and add an accessible language toggle. Invalid values are ignored and storage failures degrade to in-memory behavior.

Validation

• npx tsc --noEmit
• node --test --experimental-strip-types tests/web/app-render.test.ts
• git diff --check

Impact

• User-visible behavior: adds a language toggle and reload persistence.
• Model-visible context/tools: none.
• Runtime/lifecycle: none.
• Persisted config/data: browser-local language key only; no Session data or secrets.
• Compatibility/risk: unsupported storage values fall back to browser detection.

[tt-a1i]

需要改：语言偏好绕开了 /openpi-setup

web/ui/app.js 把 en/zh 写进 localStorage（openpi.language）。包级、用户可选的偏好必须走 extensions/setup、setup-config.ts、无参 /openpi-setup 状态、SETUP.md 和 README。#371 的主题已经按这条合同做了。

请把语言做成和 #371 一样的 snapshot 偏好，不要在浏览器里开第二份真相。切换语言 的 aria/title 写死中文也请一并改掉。