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.json

@@ -79,7 +79,8 @@
           "microphonePermission": "OpenCode uses the microphone to capture your voice when using speech-to-text input.",
           "speechRecognitionPermission": "OpenCode uses speech recognition so you can dictate prompts to your AI coding agent hands-free."
         }
-      ]
+      ],
+      "expo-localization"
     ],
     "experiments": {
       "typedRoutes": true,