Enhance model and service documentation with detailed comments and descriptions

- Updated Group, Trip, User, and other model classes to include comprehensive documentation for better understanding and maintainability.
- Improved error handling and logging in services, including AuthService, ErrorService, and StorageService.
- Added validation and business logic explanations in ExpenseService and TripService.
- Refactored method comments to follow a consistent format across the codebase.
- Translated error messages and comments from French to English for consistency.

lib/blocs/theme/theme_bloc.dart

@@ -4,23 +4,39 @@ import 'package:shared_preferences/shared_preferences.dart';
 import 'theme_event.dart';
 import 'theme_state.dart';
 
+/// BLoC for managing application theme preferences.
+/// 
+/// This BLoC handles theme-related events and manages the application's
+/// theme mode (light, dark, or system). It persists theme preferences
+/// using SharedPreferences for consistency across app sessions.
 class ThemeBloc extends Bloc<ThemeEvent, ThemeState> {
+  /// Creates a new [ThemeBloc] with default theme state.
+  /// 
+  /// Registers event handlers for theme changes and loading saved preferences.
   ThemeBloc() : super(const ThemeState()) {
     on<ThemeChanged>(_onThemeChanged);
     on<ThemeLoadRequested>(_onThemeLoadRequested);
   }
 
+  /// Handles [ThemeChanged] events.
+  /// 
+  /// Updates the theme mode and persists the preference to local storage.
+  /// This ensures the theme choice is remembered across app restarts.
   Future<void> _onThemeChanged(
     ThemeChanged event,
     Emitter<ThemeState> emit,
   ) async {
     emit(state.copyWith(themeMode: event.themeMode));
     
-    // Sauvegarder la préférence
+    // Save the theme preference to persistent storage
     final prefs = await SharedPreferences.getInstance();
     await prefs.setString('themeMode', event.themeMode.toString());
   }
 
+  /// Handles [ThemeLoadRequested] events.
+  /// 
+  /// Loads the saved theme preference from SharedPreferences and applies it.
+  /// If no preference is saved, the theme remains as system default.
   Future<void> _onThemeLoadRequested(
     ThemeLoadRequested event,
     Emitter<ThemeState> emit,