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/package-lock.json

@@ -20,6 +20,7 @@
         "@tanstack/react-query": "^5.90.2",
         "axios": "^1.12.2",
         "date-fns": "^4.1.0",
+        "date-fns-tz": "^3.2.0",
         "focus-trap-react": "^11.0.4",
         "framer-motion": "^12.23.22",
         "graphql": "^16.11.0",
@@ -6949,6 +6950,15 @@
         "url": "https://github.com/sponsors/kossnocorp"
       }
     },
+    "node_modules/date-fns-tz": {
+      "version": "3.2.0",
+      "resolved": "https://registry.npmjs.org/date-fns-tz/-/date-fns-tz-3.2.0.tgz",
+      "integrity": "sha512-sg8HqoTEulcbbbVXeg84u5UnlsQa8GS5QXMqjjYIhS4abEVVKIUwe0/l/UhrZdKaL/W5eWZNlbTeEIiOXTcsBQ==",
+      "license": "MIT",
+      "peerDependencies": {
+        "date-fns": "^3.0.0 || ^4.0.0"
+      }
+    },
     "node_modules/debug": {
       "version": "4.4.3",
       "resolved": "https://registry.npmjs.org/debug/-/debug-4.4.3.tgz",