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/fr/common.json

@@ -0,0 +1,66 @@
+{
+  "appName": "Maternal",
+  "appDescription": "Assistant de Garde d'Enfants Alimenté par l'IA",
+  "welcome": "Bienvenue",
+  "loading": "Chargement...",
+  "save": "Enregistrer",
+  "cancel": "Annuler",
+  "delete": "Supprimer",
+  "edit": "Modifier",
+  "add": "Ajouter",
+  "close": "Fermer",
+  "confirm": "Confirmer",
+  "back": "Retour",
+  "next": "Suivant",
+  "previous": "Précédent",
+  "finish": "Terminer",
+  "search": "Rechercher",
+  "filter": "Filtrer",
+  "sort": "Trier",
+  "refresh": "Actualiser",
+  "retry": "Réessayer",
+  "continue": "Continuer",
+  "submit": "Soumettre",
+  "reset": "Réinitialiser",
+  "clear": "Effacer",
+  "yes": "Oui",
+  "no": "Non",
+  "ok": "OK",
+  "error": "Erreur",
+  "success": "Succès",
+  "warning": "Avertissement",
+  "info": "Information",
+  "home": "Accueil",
+  "settings": "Paramètres",
+  "profile": "Profil",
+  "logout": "Déconnexion",
+  "login": "Connexion",
+  "signup": "S'inscrire",
+  "email": "E-mail",
+  "password": "Mot de passe",
+  "name": "Nom",
+  "date": "Date",
+  "time": "Heure",
+  "duration": "Durée",
+  "notes": "Notes",
+  "optional": "Optionnel",
+  "required": "Obligatoire",
+  "units": {
+    "metric": "Métrique",
+    "imperial": "Impérial"
+  },
+  "measurements": {
+    "weight": "Poids",
+    "height": "Taille",
+    "temperature": "Température",
+    "volume": "Volume"
+  },
+  "navigation": {
+    "home": "Accueil",
+    "tracking": "Suivi",
+    "ai": "Assistant IA",
+    "family": "Famille",
+    "insights": "Analyses",
+    "settings": "Paramètres"
+  }
+}