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/utils/unitConversion.ts

@@ -0,0 +1,184 @@
+import { MeasurementSystem } from '@/hooks/useLocale';
+
+export interface ConvertedValue {
+  value: number;
+  unit: string;
+}
+
+/**
+ * Convert weight from kilograms to the preferred measurement system
+ */
+export function convertWeight(
+  valueInKg: number,
+  system: MeasurementSystem
+): ConvertedValue {
+  if (system === 'imperial') {
+    // Convert kg to lbs (1 kg = 2.20462 lbs)
+    return {
+      value: valueInKg * 2.20462,
+      unit: 'lb',
+    };
+  }
+  return {
+    value: valueInKg,
+    unit: 'kg',
+  };
+}
+
+/**
+ * Convert weight to kilograms from the preferred measurement system
+ */
+export function convertWeightToKg(
+  value: number,
+  system: MeasurementSystem
+): number {
+  if (system === 'imperial') {
+    // Convert lbs to kg
+    return value / 2.20462;
+  }
+  return value;
+}
+
+/**
+ * Convert height from centimeters to the preferred measurement system
+ */
+export function convertHeight(
+  valueInCm: number,
+  system: MeasurementSystem
+): ConvertedValue {
+  if (system === 'imperial') {
+    // Convert cm to inches (1 cm = 0.393701 inches)
+    const inches = valueInCm * 0.393701;
+    return {
+      value: inches,
+      unit: 'in',
+    };
+  }
+  return {
+    value: valueInCm,
+    unit: 'cm',
+  };
+}
+
+/**
+ * Convert height to centimeters from the preferred measurement system
+ */
+export function convertHeightToCm(
+  value: number,
+  system: MeasurementSystem
+): number {
+  if (system === 'imperial') {
+    // Convert inches to cm
+    return value / 0.393701;
+  }
+  return value;
+}
+
+/**
+ * Convert temperature from Celsius to the preferred measurement system
+ */
+export function convertTemperature(
+  valueInCelsius: number,
+  system: MeasurementSystem
+): ConvertedValue {
+  if (system === 'imperial') {
+    // Convert Celsius to Fahrenheit: F = (C × 9/5) + 32
+    return {
+      value: (valueInCelsius * 9) / 5 + 32,
+      unit: '°F',
+    };
+  }
+  return {
+    value: valueInCelsius,
+    unit: '°C',
+  };
+}
+
+/**
+ * Convert temperature to Celsius from the preferred measurement system
+ */
+export function convertTemperatureToCelsius(
+  value: number,
+  system: MeasurementSystem
+): number {
+  if (system === 'imperial') {
+    // Convert Fahrenheit to Celsius: C = (F - 32) × 5/9
+    return ((value - 32) * 5) / 9;
+  }
+  return value;
+}
+
+/**
+ * Convert volume from milliliters to the preferred measurement system
+ */
+export function convertVolume(
+  valueInMl: number,
+  system: MeasurementSystem
+): ConvertedValue {
+  if (system === 'imperial') {
+    // Convert ml to oz (1 oz = 29.5735 ml)
+    return {
+      value: valueInMl / 29.5735,
+      unit: 'oz',
+    };
+  }
+  return {
+    value: valueInMl,
+    unit: 'ml',
+  };
+}
+
+/**
+ * Convert volume to milliliters from the preferred measurement system
+ */
+export function convertVolumeToMl(
+  value: number,
+  system: MeasurementSystem
+): number {
+  if (system === 'imperial') {
+    // Convert oz to ml
+    return value * 29.5735;
+  }
+  return value;
+}
+
+/**
+ * Get the unit symbol for a measurement type
+ */
+export function getUnitSymbol(
+  type: 'weight' | 'height' | 'temperature' | 'volume',
+  system: MeasurementSystem
+): string {
+  const units = {
+    metric: {
+      weight: 'kg',
+      height: 'cm',
+      temperature: '°C',
+      volume: 'ml',
+    },
+    imperial: {
+      weight: 'lb',
+      height: 'in',
+      temperature: '°F',
+      volume: 'oz',
+    },
+  };
+
+  return units[system][type];
+}
+
+/**
+ * Get conversion factor for a measurement type
+ */
+export function getConversionFactor(
+  type: 'weight' | 'height' | 'temperature' | 'volume'
+): { toImperial: number; toMetric: number } {
+  const factors = {
+    weight: { toImperial: 2.20462, toMetric: 1 / 2.20462 },
+    height: { toImperial: 0.393701, toMetric: 1 / 0.393701 },
+    temperature: { toImperial: 1, toMetric: 1 }, // Temperature uses formula, not factor
+    volume: { toImperial: 1 / 29.5735, toMetric: 29.5735 },
+  };
+
+  return factors[type];
+}