feat(i18n): add app locale foundation

[2233admin]

Summary

• Add minimal i18n locale foundation utilities without any UI copy migration.
• New file: ui/src/i18n/locale.ts
  • AppLocale ('en' | 'zh')
  • resolveAppLocale(): AppLocale (localStorage + fallback)
  • saveAppLocale(locale): persist
• App.tsx now initializes html lang via resolveAppLocale() so locale context can be added incrementally.

Scope

• P1 foundation only. No page/sidebar copy localization changes.

Notes

• This is prepared for downstream PRs to pick up and complete zh/en migrations.

[2233admin]

这个 PR 是 P1：app 层 i18n 基础设施（非页面文案与交互改造）。本次改动仅包含 ui/src/App.tsx 与 ui/src/i18n/locale.ts 的基础初始化逻辑，不涉及可截图的核心交互。建议你在后续 P2/P3 的 UI / 交互改动 PR 中补充截图。