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/models/expense.dart

@@ -3,47 +3,109 @@ import 'package:equatable/equatable.dart';
 import 'package:flutter/material.dart';
 import 'expense_split.dart';
 
+/// Enumeration of supported currencies for expenses.
+/// 
+/// Each currency includes both a display symbol and standard currency code.
 enum ExpenseCurrency {
+  /// Euro currency
   eur('€', 'EUR'),
+  /// US Dollar currency  
   usd('\$', 'USD'),
+  /// British Pound currency
   gbp('£', 'GBP');
 
   const ExpenseCurrency(this.symbol, this.code);
+  
+  /// Currency symbol for display (e.g., €, $, £)
   final String symbol;
+  
+  /// Standard currency code (e.g., EUR, USD, GBP)
   final String code;
 }
 
+/// Enumeration of expense categories with display names and icons.
+/// 
+/// Provides predefined categories for organizing travel expenses.
 enum ExpenseCategory {
+  /// Restaurant and food expenses
   restaurant('Restaurant', Icons.restaurant),
+  /// Transportation expenses
   transport('Transport', Icons.directions_car),
-  accommodation('Hébergement', Icons.hotel),
-  entertainment('Loisirs', Icons.local_activity),
+  /// Accommodation and lodging expenses
+  accommodation('Accommodation', Icons.hotel),
+  /// Entertainment and activity expenses
+  entertainment('Entertainment', Icons.local_activity),
+  /// Shopping expenses
   shopping('Shopping', Icons.shopping_bag),
-  other('Autre', Icons.category);
+  /// Other miscellaneous expenses
+  other('Other', Icons.category);
 
   const ExpenseCategory(this.displayName, this.icon);
+  
+  /// Human-readable display name for the category
   final String displayName;
+  
+  /// Icon representing the category
   final IconData icon;
 }
 
+/// Model representing a travel expense.
+/// 
+/// This class encapsulates all information about an expense including
+/// amount, currency, category, who paid, how it's split among participants,
+/// and receipt information. It extends [Equatable] for value comparison.
 class Expense extends Equatable {
+  /// Unique identifier for the expense
   final String id;
+  
+  /// ID of the group this expense belongs to
   final String groupId;
+  
+  /// Description of the expense
   final String description;
+  
+  /// Amount of the expense in the original currency
   final double amount;
+  
+  /// Currency of the expense
   final ExpenseCurrency currency;
-  final double amountInEur; // Montant converti en EUR
+  
+  /// Amount converted to EUR for standardized calculations
+  final double amountInEur;
+  
+  /// Category of the expense
   final ExpenseCategory category;
+  
+  /// ID of the user who paid for this expense
   final String paidById;
+  
+  /// Name of the user who paid for this expense
   final String paidByName;
+  
+  /// Date when the expense occurred
   final DateTime date;
+  
+  /// Timestamp when the expense was created
   final DateTime createdAt;
+  
+  /// Timestamp when the expense was last edited (null if never edited)
   final DateTime? editedAt;
+  
+  /// Whether this expense has been edited after creation
   final bool isEdited;
+  
+  /// Whether this expense has been archived
   final bool isArchived;
+  
+  /// URL to the receipt image (optional)
   final String? receiptUrl;
+  
+  /// List of expense splits showing how the cost is divided
   final List<ExpenseSplit> splits;
 
+  /// Creates a new [Expense] instance.
+  /// 
+  /// All parameters except [editedAt] and [receiptUrl] are required.
   const Expense({
     required this.id,
     required this.groupId,