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/lib/i18n/config.ts

@@ -0,0 +1,125 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import LanguageDetector from 'i18next-browser-languagedetector';
+
+// Import translation files
+import enCommon from '@/locales/en/common.json';
+import enTracking from '@/locales/en/tracking.json';
+import enAi from '@/locales/en/ai.json';
+import enAuth from '@/locales/en/auth.json';
+import enSettings from '@/locales/en/settings.json';
+import enOnboarding from '@/locales/en/onboarding.json';
+import enErrors from '@/locales/en/errors.json';
+
+import esCommon from '@/locales/es/common.json';
+import esTracking from '@/locales/es/tracking.json';
+import esAi from '@/locales/es/ai.json';
+import esAuth from '@/locales/es/auth.json';
+import esSettings from '@/locales/es/settings.json';
+import esOnboarding from '@/locales/es/onboarding.json';
+import esErrors from '@/locales/es/errors.json';
+
+import frCommon from '@/locales/fr/common.json';
+import frTracking from '@/locales/fr/tracking.json';
+import frAi from '@/locales/fr/ai.json';
+import frAuth from '@/locales/fr/auth.json';
+import frSettings from '@/locales/fr/settings.json';
+import frOnboarding from '@/locales/fr/onboarding.json';
+import frErrors from '@/locales/fr/errors.json';
+
+import ptCommon from '@/locales/pt/common.json';
+import ptTracking from '@/locales/pt/tracking.json';
+import ptAi from '@/locales/pt/ai.json';
+import ptAuth from '@/locales/pt/auth.json';
+import ptSettings from '@/locales/pt/settings.json';
+import ptOnboarding from '@/locales/pt/onboarding.json';
+import ptErrors from '@/locales/pt/errors.json';
+
+import zhCommon from '@/locales/zh/common.json';
+import zhTracking from '@/locales/zh/tracking.json';
+import zhAi from '@/locales/zh/ai.json';
+import zhAuth from '@/locales/zh/auth.json';
+import zhSettings from '@/locales/zh/settings.json';
+import zhOnboarding from '@/locales/zh/onboarding.json';
+import zhErrors from '@/locales/zh/errors.json';
+
+export const resources = {
+  en: {
+    common: enCommon,
+    tracking: enTracking,
+    ai: enAi,
+    auth: enAuth,
+    settings: enSettings,
+    onboarding: enOnboarding,
+    errors: enErrors,
+  },
+  es: {
+    common: esCommon,
+    tracking: esTracking,
+    ai: esAi,
+    auth: esAuth,
+    settings: esSettings,
+    onboarding: esOnboarding,
+    errors: esErrors,
+  },
+  fr: {
+    common: frCommon,
+    tracking: frTracking,
+    ai: frAi,
+    auth: frAuth,
+    settings: frSettings,
+    onboarding: frOnboarding,
+    errors: frErrors,
+  },
+  pt: {
+    common: ptCommon,
+    tracking: ptTracking,
+    ai: ptAi,
+    auth: ptAuth,
+    settings: ptSettings,
+    onboarding: ptOnboarding,
+    errors: ptErrors,
+  },
+  zh: {
+    common: zhCommon,
+    tracking: zhTracking,
+    ai: zhAi,
+    auth: zhAuth,
+    settings: zhSettings,
+    onboarding: zhOnboarding,
+    errors: zhErrors,
+  },
+} as const;
+
+export const defaultNS = 'common';
+
+export const supportedLanguages = [
+  { code: 'en', name: 'English', nativeName: 'English' },
+  { code: 'es', name: 'Spanish', nativeName: 'Español' },
+  { code: 'fr', name: 'French', nativeName: 'Français' },
+  { code: 'pt', name: 'Portuguese', nativeName: 'Português' },
+  { code: 'zh', name: 'Chinese', nativeName: '中文' },
+] as const;
+
+i18n
+  .use(LanguageDetector)
+  .use(initReactI18next)
+  .init({
+    resources,
+    defaultNS,
+    fallbackLng: 'en',
+    supportedLngs: ['en', 'es', 'fr', 'pt', 'zh'],
+    interpolation: {
+      escapeValue: false, // React already escapes values
+    },
+    detection: {
+      order: ['localStorage', 'navigator'],
+      caches: ['localStorage'],
+      lookupLocalStorage: 'i18nextLng',
+    },
+    react: {
+      useSuspense: false, // Disable suspense for SSR compatibility
+    },
+  });
+
+export default i18n;