feat: Implement frontend localization with i18n and measurement units

Implemented comprehensive frontend localization infrastructure supporting
5 languages (English, Spanish, French, Portuguese, Chinese) with measurement
unit preferences (Metric/Imperial). This lays the foundation for international
user support.

**Core Infrastructure:**
- Installed i18next, react-i18next, i18next-browser-languagedetector
- Created I18nProvider component integrated into app layout
- Configured i18next with language detection and localStorage persistence
- Created 35 translation files (5 languages × 7 namespaces)

**Translation Namespaces:**
- common: App-wide UI elements, navigation, actions
- tracking: Activity tracking (feeding, sleep, diaper, milestones)
- ai: AI assistant chat interface
- auth: Authentication flows (login, signup, password reset)
- settings: Settings and preferences
- onboarding: Onboarding flow
- errors: Error messages and validation

**Custom Hooks:**
- useTranslation: Type-safe translation wrapper
- useLocale: Language and measurement system management
- useFormatting: Date, time, number, and unit formatting

**Measurement Unit Support:**
- Created unit conversion utilities (weight, height, temperature, volume)
- Metric: kg, cm, °C, ml
- Imperial: lb, in, °F, oz
- Bidirectional conversion functions

**UI Components:**
- LanguageSelector: Dropdown to change app language
- MeasurementUnitSelector: Toggle between Metric/Imperial
- Integrated both into Settings page Preferences section

**Next Steps (Remaining):**
- Add measurement preferences to backend user schema
- Create onboarding flow with language/measurement selection
- Apply translations to existing components (dashboard, tracking forms)
- Implement multi-language AI responses
- Add professional translations (currently using basic translations)

**File Highlights:**
- lib/i18n/config.ts: i18next configuration
- hooks/useFormatting.ts: Formatting utilities with locale support
- lib/utils/unitConversion.ts: Unit conversion logic
- components/settings/*Selector.tsx: Language and measurement selectors
- locales/*/: Translation files for 5 languages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

maternal-web/locales/es/common.json

@@ -0,0 +1,66 @@
+{
+  "appName": "Maternal",
+  "appDescription": "Asistente de Cuidado Infantil con IA",
+  "welcome": "Bienvenido",
+  "loading": "Cargando...",
+  "save": "Guardar",
+  "cancel": "Cancelar",
+  "delete": "Eliminar",
+  "edit": "Editar",
+  "add": "Agregar",
+  "close": "Cerrar",
+  "confirm": "Confirmar",
+  "back": "Atrás",
+  "next": "Siguiente",
+  "previous": "Anterior",
+  "finish": "Finalizar",
+  "search": "Buscar",
+  "filter": "Filtrar",
+  "sort": "Ordenar",
+  "refresh": "Actualizar",
+  "retry": "Reintentar",
+  "continue": "Continuar",
+  "submit": "Enviar",
+  "reset": "Restablecer",
+  "clear": "Limpiar",
+  "yes": "Sí",
+  "no": "No",
+  "ok": "OK",
+  "error": "Error",
+  "success": "Éxito",
+  "warning": "Advertencia",
+  "info": "Información",
+  "home": "Inicio",
+  "settings": "Configuración",
+  "profile": "Perfil",
+  "logout": "Cerrar sesión",
+  "login": "Iniciar sesión",
+  "signup": "Registrarse",
+  "email": "Correo electrónico",
+  "password": "Contraseña",
+  "name": "Nombre",
+  "date": "Fecha",
+  "time": "Hora",
+  "duration": "Duración",
+  "notes": "Notas",
+  "optional": "Opcional",
+  "required": "Obligatorio",
+  "units": {
+    "metric": "Métrico",
+    "imperial": "Imperial"
+  },
+  "measurements": {
+    "weight": "Peso",
+    "height": "Altura",
+    "temperature": "Temperatura",
+    "volume": "Volumen"
+  },
+  "navigation": {
+    "home": "Inicio",
+    "tracking": "Seguimiento",
+    "ai": "Asistente IA",
+    "family": "Familia",
+    "insights": "Análisis",
+    "settings": "Configuración"
+  }
+}