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>

app/_layout.tsx

@@ -5,6 +5,8 @@ import { useColorScheme, View, ActivityIndicator } from "react-native"
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query"
 import { GestureHandlerRootView } from "react-native-gesture-handler"
 import { BottomSheetModalProvider } from "@gorhom/bottom-sheet"
+import { I18nextProvider } from "react-i18next"
+import i18n from "../src/lib/i18n/config"
 import { useAuth } from "../src/stores/auth"
 import { useConnections } from "../src/stores/connections"
 import { useEvents } from "../src/stores/events"
@@ -107,6 +109,7 @@ function RootLayout() {
 
   return (
     <ErrorBoundary>
+      <I18nextProvider i18n={i18n}>
       <GestureHandlerRootView style={{ flex: 1 }}>
         <BottomSheetModalProvider>
           <QueryClientProvider client={queryClient}>
@@ -162,6 +165,7 @@ function RootLayout() {
           setConsentState("decided")
         }}
       />
+      </I18nextProvider>
     </ErrorBoundary>
   )
 }