fix(chat): Correction race condition + Implémentation TODOs

## Corrections Critiques

### Race Condition - Statuts de Messages
- Fix : Les icônes de statut (✓, ✓✓, ✓✓ bleu) ne s'affichaient pas
- Cause : WebSocket delivery confirmations arrivaient avant messages locaux
- Solution : Pattern Optimistic UI dans chat_bloc.dart
  - Création message temporaire immédiate
  - Ajout à la liste AVANT requête HTTP
  - Remplacement par message serveur à la réponse
- Fichier : lib/presentation/state_management/chat_bloc.dart

## Implémentation TODOs (13/21)

### Social (social_header_widget.dart)
- ✅ Copier lien du post dans presse-papiers
- ✅ Partage natif via Share.share()
- ✅ Dialogue de signalement avec 5 raisons

### Partage (share_post_dialog.dart)
- ✅ Interface sélection d'amis avec checkboxes
- ✅ Partage externe via Share API

### Média (media_upload_service.dart)
- ✅ Parsing JSON réponse backend
- ✅ Méthode deleteMedia() pour suppression
- ✅ Génération miniature vidéo

### Posts (create_post_dialog.dart, edit_post_dialog.dart)
- ✅ Extraction URL depuis uploads
- ✅ Documentation chargement médias

### Chat (conversations_screen.dart)
- ✅ Navigation vers notifications
- ✅ ConversationSearchDelegate pour recherche

## Nouveaux Fichiers

### Configuration
- build-prod.ps1 : Script build production avec dart-define
- lib/core/constants/env_config.dart : Gestion environnements

### Documentation
- TODOS_IMPLEMENTED.md : Documentation complète TODOs

## Améliorations

### Architecture
- Refactoring injection de dépendances
- Amélioration routing et navigation
- Optimisation providers (UserProvider, FriendsProvider)

### UI/UX
- Amélioration thème et couleurs
- Optimisation animations
- Meilleure gestion erreurs

### Services
- Configuration API avec env_config
- Amélioration datasources (events, users)
- Optimisation modèles de données

lib/data/models/chat_message_model.dart

@@ -0,0 +1,136 @@
+import '../../domain/entities/chat_message.dart';
+
+/// Modèle de données pour les messages de chat (Data Transfer Object).
+class ChatMessageModel {
+  ChatMessageModel({
+    required this.id,
+    required this.conversationId,
+    required this.senderId,
+    required this.senderFirstName,
+    required this.senderLastName,
+    this.senderProfileImageUrl,
+    required this.content,
+    required this.timestamp,
+    required this.isRead,
+    this.isDelivered = false,
+    this.attachmentUrl,
+    this.attachmentType,
+  });
+
+  /// Factory pour créer un [ChatMessageModel] à partir d'un JSON.
+  factory ChatMessageModel.fromJson(Map<String, dynamic> json) {
+    return ChatMessageModel(
+      id: _parseId(json, 'id', ''),
+      conversationId: _parseString(json, 'conversationId', ''),
+      senderId: _parseId(json, 'senderId', ''),
+      senderFirstName: _parseString(json, 'senderFirstName', ''),
+      senderLastName: _parseString(json, 'senderLastName', ''),
+      senderProfileImageUrl: json['senderProfileImageUrl'] as String?,
+      content: _parseString(json, 'content', ''),
+      timestamp: DateTime.parse(json['timestamp'] as String),
+      isRead: json['isRead'] as bool? ?? false,
+      isDelivered: json['isDelivered'] as bool? ?? false,
+      attachmentUrl: json['attachmentUrl'] as String?,
+      attachmentType: _parseAttachmentType(json['attachmentType'] as String?),
+    );
+  }
+
+  /// Factory pour créer un [ChatMessageModel] à partir d'une entité.
+  factory ChatMessageModel.fromEntity(ChatMessage message) {
+    return ChatMessageModel(
+      id: message.id,
+      conversationId: message.conversationId,
+      senderId: message.senderId,
+      senderFirstName: message.senderFirstName,
+      senderLastName: message.senderLastName,
+      senderProfileImageUrl: message.senderProfileImageUrl,
+      content: message.content,
+      timestamp: message.timestamp,
+      isRead: message.isRead,
+      isDelivered: message.isDelivered,
+      attachmentUrl: message.attachmentUrl,
+      attachmentType: message.attachmentType,
+    );
+  }
+
+  final String id;
+  final String conversationId;
+  final String senderId;
+  final String senderFirstName;
+  final String senderLastName;
+  final String? senderProfileImageUrl;
+  final String content;
+  final DateTime timestamp;
+  final bool isRead;
+  final bool isDelivered;
+  final String? attachmentUrl;
+  final AttachmentType? attachmentType;
+
+  /// Convertit ce modèle en JSON pour l'envoi vers l'API.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'conversationId': conversationId,
+      'senderId': senderId,
+      'senderFirstName': senderFirstName,
+      'senderLastName': senderLastName,
+      if (senderProfileImageUrl != null) 'senderProfileImageUrl': senderProfileImageUrl,
+      'content': content,
+      'timestamp': timestamp.toIso8601String(),
+      'isRead': isRead,
+      'isDelivered': isDelivered,
+      if (attachmentUrl != null) 'attachmentUrl': attachmentUrl,
+      if (attachmentType != null) 'attachmentType': _attachmentTypeToString(attachmentType!),
+    };
+  }
+
+  /// Convertit ce modèle vers une entité de domaine [ChatMessage].
+  ChatMessage toEntity() {
+    return ChatMessage(
+      id: id,
+      conversationId: conversationId,
+      senderId: senderId,
+      senderFirstName: senderFirstName,
+      senderLastName: senderLastName,
+      senderProfileImageUrl: senderProfileImageUrl,
+      content: content,
+      timestamp: timestamp,
+      isRead: isRead,
+      isDelivered: isDelivered,
+      attachmentUrl: attachmentUrl,
+      attachmentType: attachmentType,
+    );
+  }
+
+  // Méthodes de parsing
+  static String _parseString(Map<String, dynamic> json, String key, String defaultValue) {
+    return json[key] as String? ?? defaultValue;
+  }
+
+  static String _parseId(Map<String, dynamic> json, String key, String defaultValue) {
+    final value = json[key];
+    if (value == null) return defaultValue;
+    return value.toString();
+  }
+
+  static AttachmentType? _parseAttachmentType(String? type) {
+    if (type == null) return null;
+
+    switch (type.toLowerCase()) {
+      case 'image':
+        return AttachmentType.image;
+      case 'video':
+        return AttachmentType.video;
+      case 'audio':
+        return AttachmentType.audio;
+      case 'file':
+        return AttachmentType.file;
+      default:
+        return null;
+    }
+  }
+
+  static String _attachmentTypeToString(AttachmentType type) {
+    return type.toString().split('.').last;
+  }
+}