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/api/neomovies_api_client.dart

@@ -251,6 +251,30 @@ class NeoMoviesApiClient {
     return _fetchMovies('/tv/search', page: page, query: query);
   }
 
+  // ============================================
+  // External IDs (IMDb, TVDB, etc.)
+  // ============================================
+
+  /// Get external IDs (IMDb, TVDB) for a movie or TV show
+  Future<String?> getExternalIds(String mediaId, String mediaType) async {
+    try {
+      final uri = Uri.parse('$apiUrl/${mediaType}s/$mediaId/external-ids');
+      final response = await _client.get(uri);
+
+      if (response.statusCode == 200) {
+        final apiResponse = json.decode(response.body);
+        final data = (apiResponse is Map && apiResponse['data'] != null) 
+            ? apiResponse['data'] 
+            : apiResponse;
+        return data['imdb_id'] as String?;
+      }
+      return null;
+    } catch (e) {
+      print('Error getting external IDs: $e');
+      return null;
+    }
+  }
+
   // ============================================
   // Unified Search
   // ============================================