feat: Complete Phase 1 - Full Flutter app implementation with comprehensive features

Version: 1.1.0

Major changes:
- Implemented complete Flutter app structure with all core features
- Added comprehensive UI screens for auth, countdown, goals, profile, settings, and social features
- Integrated Supabase backend with authentication and data repositories
- Added offline support with Hive caching and local storage
- Implemented comprehensive routing with go_router
- Added location services with Google Maps integration
- Implemented notifications and home widget support
- Added voice recording capabilities and AI chat features
- Created comprehensive test suite and documentation
- Added Android and iOS platform configurations
- Implemented achievements system and social features
- Added calendar integration and bucket list functionality

This represents a complete Phase 1 milestone with 3,775 additions across 31 files.

lifetimer/lib/data/repositories/countdown_repository.dart

@@ -10,6 +10,11 @@ class CountdownRepository {
 
   Future<app.User> startCountdown(String userId) async {
     try {
+      final user = await getCountdownInfo(userId);
+      if (user.countdownStartDate != null) {
+        throw const ValidationFailure('Countdown has already started and cannot be restarted');
+      }
+
       final startDate = DateTime.now();
       final endDate = DateTimeUtils.calculateEndDate(startDate);