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

lib/models/message.dart

@@ -10,6 +10,7 @@ class Message {
   final Map<String, String> reactions; // userId -> emoji
   final DateTime? editedAt;
   final bool isEdited;
+  final bool isDeleted;
 
   Message({
     this.id = '',
@@ -21,6 +22,7 @@ class Message {
     this.reactions = const {},
     this.editedAt,
     this.isEdited = false,
+    this.isDeleted = false,
   });
 
   factory Message.fromFirestore(DocumentSnapshot doc) {
@@ -39,6 +41,7 @@ class Message {
       reactions: reactionsData?.map((key, value) => MapEntry(key, value.toString())) ?? {},
       editedAt: editedAtTimestamp?.toDate(),
       isEdited: data['isEdited'] ?? false,
+      isDeleted: data['isDeleted'] ?? false,
     );
   }
 
@@ -52,6 +55,7 @@ class Message {
       'reactions': reactions,
       'editedAt': editedAt != null ? Timestamp.fromDate(editedAt!) : null,
       'isEdited': isEdited,
+      'isDeleted': isDeleted,
     };
   }
 
@@ -65,6 +69,7 @@ class Message {
     Map<String, String>? reactions,
     DateTime? editedAt,
     bool? isEdited,
+    bool? isDeleted,
   }) {
     return Message(
       id: id ?? this.id,
@@ -76,6 +81,7 @@ class Message {
       reactions: reactions ?? this.reactions,
       editedAt: editedAt ?? this.editedAt,
       isEdited: isEdited ?? this.isEdited,
+      isDeleted: isDeleted ?? this.isDeleted,
     );
   }
 }