fix: make PlexMetadata type/key/title nullable for folder items

Plex API returns Directory entries without type field, causing
TypeError crash in fromJson. Made fields nullable and migrated
callers to use mediaType enum and displayTitle getter instead
of raw string access.

lib/providers/download_provider.dart

@@ -515,10 +515,10 @@ class DownloadProvider extends ChangeNotifier {
     }
 
     // We have metadata, check type
-    final type = meta.type.toLowerCase();
-    if (type == 'show') {
+    final mt = meta.mediaType;
+    if (mt == PlexMediaType.show) {
       return getAggregateProgressForShow(serverId, ratingKey);
-    } else if (type == 'season') {
+    } else if (mt == PlexMediaType.season) {
       return getAggregateProgressForSeason(serverId, ratingKey);
     }
 
@@ -606,19 +606,19 @@ class DownloadProvider extends ChangeNotifier {
       _queueing.add(globalKey);
       notifyListeners();
 
-      final type = metadata.type.toLowerCase();
+      final mt = metadata.mediaType;
 
-      if (type == 'movie' || type == 'episode') {
+      if (mt == PlexMediaType.movie || mt == PlexMediaType.episode) {
         // Direct download of a single item
         await _queueSingleDownload(metadata, client);
         return 1;
-      } else if (type == 'show') {
+      } else if (mt == PlexMediaType.show) {
         // Store show metadata so getProgress() can identify it as a show
         _metadata[globalKey] = metadata;
 
         // Download all episodes from all seasons
         return await _queueShowDownload(metadata, client);
-      } else if (type == 'season') {
+      } else if (mt == PlexMediaType.season) {
         // Store season metadata so getProgress() can identify it as a season
         _metadata[globalKey] = metadata;
 
@@ -765,11 +765,11 @@ class DownloadProvider extends ChangeNotifier {
   /// Used for resuming partial downloads
   /// Returns the number of episodes queued
   Future<int> queueMissingEpisodes(PlexMetadata metadata, PlexClient client) async {
-    final type = metadata.type.toLowerCase();
+    final mt = metadata.mediaType;
 
-    if (type == 'show') {
+    if (mt == PlexMediaType.show) {
       return await _queueMissingShowEpisodes(metadata, client);
-    } else if (type == 'season') {
+    } else if (mt == PlexMediaType.season) {
       return await _queueMissingSeasonEpisodes(metadata, client);
     } else {
       throw Exception('queueMissingEpisodes only supports shows/seasons');