feat(i18n): add i18n infrastructure + Simplified Chinese, localize Settings screen (refs #68) (#97)

- Add expo-localization, i18next, react-i18next (versions aligned with
  Expo 54 / RN 0.81)
- src/lib/i18n/locale-resolve.ts: pure locale-resolution helpers
  (system tag -> supported catalog, with en fallback), unit-tested via
  node --test with no RN imports
- src/lib/i18n/config.ts: i18next init wired to expo-localization
  device detection, en.json + zh-Hans.json catalogs
- Persist a locale preference (system | en | zh-Hans) in the settings
  zustand store, applied immediately via i18next.changeLanguage
- Wire I18nextProvider in app/_layout.tsx
- Localize the Settings screen (~28 strings) as the reference pattern
  for extracting user-facing strings, with a language picker row and
  reviewed Simplified Chinese translations

Other screens (session/[id], connection/*, index, chat components)
are deferred follow-up — issue #68 stays open for that work.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -26,14 +26,17 @@
     "expo-image-picker": "17.0.10",
     "expo-linking": "~8.0.11",
     "expo-local-authentication": "~17.0.8",
+    "expo-localization": "~17.0.9",
     "expo-notifications": "~0.32.16",
     "expo-router": "~6.0.22",
     "expo-secure-store": "~15.0.8",
     "expo-speech-recognition": "3.0.1",
     "expo-status-bar": "~3.0.9",
     "expo-store-review": "~9.0.9",
+    "i18next": "^26.3.6",
     "posthog-react-native": "^4.57.0",
     "react": "19.1.0",
+    "react-i18next": "^17.0.10",
     "react-native": "0.81.5",
     "react-native-gesture-handler": "~2.28.0",
     "react-native-marked": "8.0.0",