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,

lib/blocs/theme/theme_event.dart

@@ -1,20 +1,35 @@
 import 'package:equatable/equatable.dart';
 import 'package:flutter/material.dart';
 
+/// Abstract base class for all theme-related events.
+/// 
+/// This class extends [Equatable] to enable value equality for event comparison.
+/// All theme events in the application should inherit from this class.
 abstract class ThemeEvent extends Equatable {
+  /// Creates a new [ThemeEvent].
   const ThemeEvent();
 
   @override
   List<Object?> get props => [];
 }
 
+/// Event to request a theme change.
+/// 
+/// This event is dispatched when the user wants to change the application theme
+/// mode (light, dark, or system). The new theme mode is persisted and applied immediately.
 class ThemeChanged extends ThemeEvent {
+  /// The new theme mode to apply.
   final ThemeMode themeMode;
 
+  /// Creates a new [ThemeChanged] event with the specified [themeMode].
   const ThemeChanged({required this.themeMode});
 
   @override
   List<Object?> get props => [themeMode];
 }
 
+/// Event to request loading of saved theme preferences.
+/// 
+/// This event is typically dispatched when the app starts to restore
+/// the user's previously selected theme preference.
 class ThemeLoadRequested extends ThemeEvent {}

lib/blocs/theme/theme_state.dart

@@ -1,15 +1,32 @@
 import 'package:equatable/equatable.dart';
 import 'package:flutter/material.dart';
 
+/// State class for theme management.
+/// 
+/// This class represents the current theme state of the application,
+/// including the selected theme mode and provides utility methods
+/// for theme-related operations.
 class ThemeState extends Equatable {
+  /// The current theme mode of the application.
   final ThemeMode themeMode;
 
+  /// Creates a new [ThemeState] with the specified [themeMode].
+  /// 
+  /// Defaults to [ThemeMode.system] if no theme mode is provided.
   const ThemeState({this.themeMode = ThemeMode.system});
 
+  /// Whether the current theme mode is explicitly set to dark.
+  /// 
+  /// Returns true only if the theme mode is [ThemeMode.dark].
+  /// Returns false for [ThemeMode.light] and [ThemeMode.system].
   bool get isDarkMode {
     return themeMode == ThemeMode.dark;
   }
 
+  /// Creates a copy of this state with optionally modified properties.
+  /// 
+  /// Allows updating the theme mode while preserving other state properties.
+  /// Useful for state transitions in the theme BLoC.
   ThemeState copyWith({ThemeMode? themeMode}) {
     return ThemeState(
       themeMode: themeMode ?? this.themeMode,