Introduce a lightweight React context-based i18n layer for the desktop
app and translate the UI into Simplified Chinese.
- New apps/desktop/src/i18n module: typed Translations interface, en + zh
locale tables, I18nProvider/useI18n, localStorage-persisted locale
(defaults to English), and language endonym metadata for the picker.
- Wire I18nProvider at the app root in main.tsx.
- Refactor 24 desktop screens/components to read strings from the `t`
object instead of hard-coded English.
- Add a unit test for the i18n context.
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