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_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 {}