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_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,