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/trip.dart

@@ -1,20 +1,52 @@
 import 'dart:convert';
 import 'package:cloud_firestore/cloud_firestore.dart';
 
+/// Model representing a travel trip in the application.
+/// 
+/// This class encapsulates all trip-related information including dates,
+/// location, participants, and budget details. It provides serialization
+/// methods for Firebase operations and supports trip lifecycle management.
 class Trip {
+  /// Unique identifier for the trip (usually Firestore document ID).
   final String? id;
+  
+  /// Title or name of the trip.
   final String title;
+  
+  /// Detailed description of the trip.
   final String description;
+  
+  /// Trip destination or location.
   final String location;
+  
+  /// Trip start date and time.
   final DateTime startDate;
+  
+  /// Trip end date and time.
   final DateTime endDate;
+  
+  /// Optional budget for the trip in the local currency.
   final double? budget;
+  
+  /// List of participant user IDs.
   final List<String> participants;
+  
+  /// User ID of the trip creator.
   final String createdBy;
+  
+  /// Timestamp when the trip was created.
   final DateTime createdAt;
+  
+  /// Timestamp when the trip was last updated.
   final DateTime updatedAt;
+  
+  /// Current status of the trip (e.g., 'draft', 'active', 'completed').
   final String status;
 
+  /// Creates a new [Trip] instance.
+  /// 
+  /// Most fields are required except [id] and [budget].
+  /// [status] defaults to 'draft' for new trips.
   Trip({
     this.id,
     required this.title,