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/group.dart

@@ -1,14 +1,37 @@
 import 'group_member.dart';
 
+/// Model representing a travel group.
+/// 
+/// A group is a collection of travelers who are part of a specific trip.
+/// It contains information about group members, creation details, and
+/// provides methods for serialization with Firestore.
 class Group {
+  /// Unique identifier for the group
   final String id; 
+  
+  /// Display name of the group
   final String name;
+  
+  /// ID of the trip this group belongs to
   final String tripId;
+  
+  /// ID of the user who created this group
   final String createdBy;
+  
+  /// Timestamp when the group was created
   final DateTime createdAt;
+  
+  /// Timestamp when the group was last updated
   final DateTime updatedAt;
+  
+  /// List of members in this group
   final List<GroupMember> members;
 
+  /// Creates a new [Group] instance.
+  /// 
+  /// [id], [name], [tripId], and [createdBy] are required.
+  /// [createdAt] and [updatedAt] default to current time if not provided.
+  /// [members] defaults to empty list if not provided.
   Group({
     required this.id, 
     required this.name,
@@ -21,6 +44,12 @@ class Group {
         updatedAt = updatedAt ?? DateTime.now(),
         members = members ?? [];
 
+  /// Creates a [Group] instance from a Firestore document map.
+  /// 
+  /// [map] - The document data from Firestore
+  /// [id] - The document ID from Firestore
+  /// 
+  /// Returns a new [Group] instance with data from the map.
   factory Group.fromMap(Map<String, dynamic> map, String id) {
     return Group(
       id: id,