fix: add Downloads screen to navigation and fix API models

1. Add Downloads screen to main navigation:
   - Import DownloadsScreen in main_screen.dart
   - Replace placeholder 'Downloads Page' with actual DownloadsScreen component
   - Downloads tab now fully functional with torrent management

2. Fix authentication models for API compatibility:
   - AuthResponse: Handle wrapped API response with 'data' field
   - User model: Add 'verified' field, support both '_id' and 'id' fields
   - Add toJson() method to User for serialization
   - Fix parsing to match backend response format

3. Fix movie details screen (gray screen issue):
   - Implement getExternalIds() in NeoMoviesApiClient
   - Add IMDb ID fetching via /movies/{id}/external-ids endpoint
   - Update api_client.dart to use new getExternalIds method
   - Fix movie detail provider to properly load IMDb IDs

Changes:
- lib/presentation/screens/main_screen.dart: Add DownloadsScreen import and replace placeholder
- lib/data/models/auth_response.dart: Handle wrapped 'data' response
- lib/data/models/user.dart: Add verified field and toJson method
- lib/data/api/neomovies_api_client.dart: Add getExternalIds endpoint
- lib/data/api/api_client.dart: Implement getImdbId using external IDs

Result:
✅ Downloads tab works and shows torrent list
✅ Authentication properly parses API responses
✅ Movie detail screen loads IMDb IDs correctly
✅ All API models match backend format

lib/data/models/user.dart

@@ -2,14 +2,30 @@ class User {
   final String id;
   final String name;
   final String email;
+  final bool verified;
 
-  User({required this.id, required this.name, required this.email});
+  User({
+    required this.id, 
+    required this.name, 
+    required this.email,
+    this.verified = true,
+  });
 
   factory User.fromJson(Map<String, dynamic> json) {
     return User(
-      id: json['_id'] as String? ?? '',
+      id: (json['_id'] ?? json['id'] ?? '') as String,
       name: json['name'] as String? ?? '',
       email: json['email'] as String? ?? '',
+      verified: json['verified'] as bool? ?? true,
     );
   }
+  
+  Map<String, dynamic> toJson() {
+    return {
+      '_id': id,
+      'name': name,
+      'email': email,
+      'verified': verified,
+    };
+  }
 }