Implement message deletion functionality: add isDeleted flag to Message model, update deleteMessage method in MessageRepository, and adjust chat display for deleted messages.

lib/components/group/chat_group_content.dart

@@ -476,8 +476,12 @@ class _ChatGroupContentState extends State<ChatGroupContent> {
                       const SizedBox(height: 4),
                     ],
                     Text(
-                      message.text,
-                      style: TextStyle(fontSize: 15, color: textColor),
+                      message.isDeleted ? 'a supprimé un message' : message.text,
+                      style: TextStyle(
+                        fontSize: 15,
+                        color: message.isDeleted ? textColor.withValues(alpha: 0.5) : textColor,
+                        fontStyle: message.isDeleted ? FontStyle.italic : FontStyle.normal,
+                      ),
                     ),
                     const SizedBox(height: 4),
                     Row(

lib/components/group/group_content.dart

@@ -127,17 +127,6 @@ class _GroupContentState extends State<GroupContent> {
       child: ListView(
         padding: const EdgeInsets.all(16),
         children: [
-          const Text(
-            'Mes groupes',
-            style: TextStyle(fontSize: 28, fontWeight: FontWeight.bold),
-          ),
-          const SizedBox(height: 8),
-          Text(
-            'Discutez avec les participants',
-            style: TextStyle(fontSize: 14, color: Colors.grey[600]),
-          ),
-          const SizedBox(height: 24),
-
           ...groups.map((group) {
             return Padding(
               padding: const EdgeInsets.only(bottom: 12),