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/components/settings/MeasurementUnitSelector.tsx

@@ -0,0 +1,79 @@
+'use client';
+
+import { useState, useEffect } from 'react';
+import {
+  FormControl,
+  InputLabel,
+  Select,
+  MenuItem,
+  SelectChangeEvent,
+  Box,
+  Typography,
+} from '@mui/material';
+import { useLocale, MeasurementSystem } from '@/hooks/useLocale';
+import { useTranslation } from '@/hooks/useTranslation';
+
+interface MeasurementUnitSelectorProps {
+  variant?: 'standard' | 'outlined' | 'filled';
+  fullWidth?: boolean;
+  showDescription?: boolean;
+}
+
+/**
+ * Measurement unit selector component for choosing between Metric and Imperial systems
+ */
+export function MeasurementUnitSelector({
+  variant = 'outlined',
+  fullWidth = true,
+  showDescription = true,
+}: MeasurementUnitSelectorProps) {
+  const { measurementSystem, setMeasurementSystem } = useLocale();
+  const { t } = useTranslation('settings');
+  const [selectedSystem, setSelectedSystem] = useState<MeasurementSystem>(measurementSystem);
+
+  useEffect(() => {
+    setSelectedSystem(measurementSystem);
+  }, [measurementSystem]);
+
+  const handleSystemChange = (event: SelectChangeEvent) => {
+    const newSystem = event.target.value as MeasurementSystem;
+    setSelectedSystem(newSystem);
+    setMeasurementSystem(newSystem);
+  };
+
+  return (
+    <FormControl variant={variant} fullWidth={fullWidth}>
+      <InputLabel id="measurement-unit-selector-label">
+        {t('preferences.measurementUnits')}
+      </InputLabel>
+      <Select
+        labelId="measurement-unit-selector-label"
+        id="measurement-unit-selector"
+        value={selectedSystem}
+        label={t('preferences.measurementUnits')}
+        onChange={handleSystemChange}
+      >
+        <MenuItem value="metric">
+          <Box>
+            <Typography variant="body1">{t('preferences.metric')}</Typography>
+            {showDescription && (
+              <Typography variant="caption" color="text.secondary">
+                kg, cm, °C, ml
+              </Typography>
+            )}
+          </Box>
+        </MenuItem>
+        <MenuItem value="imperial">
+          <Box>
+            <Typography variant="body1">{t('preferences.imperial')}</Typography>
+            {showDescription && (
+              <Typography variant="caption" color="text.secondary">
+                lb, in, °F, oz
+              </Typography>
+            )}
+          </Box>
+        </MenuItem>
+      </Select>
+    </FormControl>
+  );
+}