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/balance/balance_state.dart

@@ -2,21 +2,48 @@ import 'package:equatable/equatable.dart';
 import '../../models/settlement.dart';
 import '../../models/user_balance.dart';
 
+/// Abstract base class for all balance-related states.
+/// 
+/// This class extends [Equatable] to enable value equality for state comparison.
+/// All balance states in the application should inherit from this class.
 abstract class BalanceState extends Equatable {
+  /// Creates a new [BalanceState].
   const BalanceState();
 
   @override
   List<Object?> get props => [];
 }
 
+/// Initial state of the balance bloc.
+/// 
+/// This state represents the initial state before any balance
+/// operations have been performed.
 class BalanceInitial extends BalanceState {}
 
+/// State indicating that a balance operation is in progress.
+/// 
+/// This state is used to show loading indicators during balance
+/// calculations, data loading, or settlement operations.
 class BalanceLoading extends BalanceState {}
 
+/// State indicating that group balances have been successfully loaded.
+/// 
+/// This state contains the calculated balances for all group members
+/// and the list of settlements needed to balance all debts.
 class GroupBalancesLoaded extends BalanceState {
+  /// List of user balances showing how much each user owes or is owed.
+  /// 
+  /// Positive balances indicate the user is owed money,
+  /// negative balances indicate the user owes money.
   final List<UserBalance> balances;
+  
+  /// List of settlements required to balance all debts in the group.
+  /// 
+  /// Each settlement represents a payment that needs to be made
+  /// from one user to another to settle shared expenses.
   final List<Settlement> settlements;
   
+  /// Creates a [GroupBalancesLoaded] state with the calculated data.
   const GroupBalancesLoaded({
     required this.balances,
     required this.settlements,
@@ -26,18 +53,30 @@ class GroupBalancesLoaded extends BalanceState {
   List<Object> get props => [balances, settlements];
 }
 
+/// State indicating that a balance operation has completed successfully.
+/// 
+/// This state is used to show success messages after operations like
+/// marking settlements as completed or refreshing balance calculations.
 class BalanceOperationSuccess extends BalanceState {
+  /// Success message to display to the user.
   final String message;
 
+  /// Creates a [BalanceOperationSuccess] state with the given [message].
   const BalanceOperationSuccess(this.message);
 
   @override
   List<Object?> get props => [message];
 }
 
+/// State indicating that a balance operation has failed.
+/// 
+/// This state contains an error message that can be displayed to the user
+/// when balance operations fail.
 class BalanceError extends BalanceState {
+  /// The error message describing what went wrong.
   final String message;
 
+  /// Creates a [BalanceError] state with the given error [message].
   const BalanceError(this.message);
 
   @override