Refactor code structure for improved readability and maintainability

Co-authored-by: Copilot <copilot@github.com>

flutter/lib/features/admin/domain/user_admin.dart

@@ -0,0 +1,40 @@
+/// Model for a user as returned by the admin endpoint.
+class UserAdmin {
+  final int id;
+  final String username;
+  final String email;
+  final String? firstName;
+  final String? lastName;
+  final String role;
+  final bool isPremium;
+  final DateTime? createdAt;
+
+  const UserAdmin({
+    required this.id,
+    required this.username,
+    required this.email,
+    this.firstName,
+    this.lastName,
+    required this.role,
+    required this.isPremium,
+    this.createdAt,
+  });
+
+  factory UserAdmin.fromJson(Map<String, dynamic> json) => UserAdmin(
+        id: json['id'] as int,
+        username: json['username'] as String,
+        email: json['email'] as String? ?? '',
+        firstName: json['firstName'] as String?,
+        lastName: json['lastName'] as String?,
+        role: json['role'] as String? ?? 'user',
+        isPremium: json['isPremium'] as bool? ?? false,
+        createdAt: json['createdAt'] != null ? DateTime.tryParse(json['createdAt'] as String) : null,
+      );
+
+  bool get isAdmin => role == 'admin';
+
+  String get displayName {
+    final parts = [firstName, lastName].where((s) => s != null && s.isNotEmpty).toList();
+    return parts.isNotEmpty ? parts.join(' ') : username;
+  }
+}