feat: Add timezone and time format preferences with auto-detection

This commit implements comprehensive timezone and time format customization:

## Backend Changes
- Added timeFormat field ('12h' | '24h') to user preferences JSONB in user entity
- Timezone field already existed in user entity, now actively used
- Backend ready to accept timezone on registration

## Frontend Components (2 new)
- TimeZoneSelector: Dropdown with timezones grouped by region (Americas, Europe, Asia, Pacific, Africa)
  * Auto-detect button to detect browser timezone
  * Save functionality with success/error feedback
  * Integrated into Settings > Preferences section
- TimeFormatSelector: Radio buttons to choose 12h vs 24h format
  * Live preview showing current time in selected format
  * Save functionality with user feedback
  * Integrated into Settings > Preferences section

## Timezone Auto-Detection
- Register function now auto-detects user's timezone via Intl.DateTimeFormat()
- Detected timezone sent to backend during registration
- Timezone stored in user profile for persistent preference

## Enhanced useLocalizedDate Hook
- Added useAuth integration to access user timezone and timeFormat preferences
- Installed and integrated date-fns-tz for timezone conversion
- New format() function with timezone support via useTimezone option
- New formatTime() function respecting user's 12h/24h preference
- New formatDateTime() function combining date, time, and timezone
- All formatting now respects user's:
  * Language (existing: en, es, fr, pt-BR, zh-CN)
  * Timezone (user-selected or auto-detected)
  * Time format (12h with AM/PM or 24h)

## Settings Page Updates
- Added TimeZoneSelector to Preferences card
- Added TimeFormatSelector to Preferences card
- Visual separators (Dividers) between preference sections
- Settings now show: Language | Units | Timezone | Time Format

## Translations
- Enhanced settings.json with timezone and time format keys:
  * preferences.timezone, autoDetectTimezone, timezoneUpdated
  * preferences.12hour, 24hour, timeFormatUpdated

## User Experience Flow
1. User registers → timezone auto-detected and saved
2. User can change timezone in Settings > Preferences > Time Zone
3. User can change time format in Settings > Preferences > Time Format
4. All dates/times throughout app respect these preferences
5. Changes persist across sessions

Files changed: 10 files
New dependencies: date-fns-tz

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

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

maternal-web/lib/auth/AuthContext.tsx

@@ -141,10 +141,14 @@ export const AuthProvider = ({ children }: { children: ReactNode }) => {
         osVersion: navigator.platform,
       };
 
+      // Auto-detect timezone from user's device
+      const detectedTimezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+
       const response = await apiClient.post('/api/v1/auth/register', {
         email: data.email,
         password: data.password,
         name: data.name,
+        timezone: detectedTimezone || 'UTC',
         deviceInfo,
       });