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/api_client.dart

@@ -102,10 +102,7 @@ class ApiClient {
 
   // ---- External IDs (IMDb) ----
   Future<String?> getImdbId(String mediaId, String mediaType) async {
-    // This would need to be implemented in NeoMoviesApiClient
-    // For now, return null or implement a stub
-    // TODO: Add getExternalIds endpoint to backend
-    return null;
+    return _neoClient.getExternalIds(mediaId, mediaType);
   }
 
   // ---- Auth ----