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/utils/content_utils.dart

@@ -86,7 +86,7 @@ String formatContentRating(String? contentRating) {
 
 /// Extension on PlexMetadata for type checking convenience methods
 extension PlexMetadataType on PlexMetadata {
-  String get _lowerType => type.toLowerCase();
+  String get _lowerType => type?.toLowerCase() ?? '';
 
   bool get isShow => _lowerType == ContentTypes.show;
   bool get isMovie => _lowerType == ContentTypes.movie;

lib/utils/deletion_notifier.dart

@@ -80,7 +80,7 @@ class DeletionNotifier extends BaseNotifier<DeletionEvent> {
         ratingKey: metadata.ratingKey,
         serverId: metadata.serverId ?? '',
         parentChain: _buildParentChain(metadata),
-        mediaType: metadata.type,
+        mediaType: metadata.type ?? '',
         leafCount: metadata.leafCount ?? 1,
         isDownloadOnly: isDownloadOnly,
       ),

lib/utils/watch_state_notifier.dart

@@ -86,7 +86,7 @@ class WatchStateNotifier extends BaseNotifier<WatchStateEvent> {
         serverId: metadata.serverId ?? '',
         changeType: isNowWatched ? WatchStateChangeType.watched : WatchStateChangeType.unwatched,
         parentChain: _buildParentChain(metadata),
-        mediaType: metadata.type,
+        mediaType: metadata.type ?? '',
         isNowWatched: isNowWatched,
       ),
     );
@@ -103,7 +103,7 @@ class WatchStateNotifier extends BaseNotifier<WatchStateEvent> {
         serverId: metadata.serverId ?? '',
         changeType: WatchStateChangeType.progressUpdate,
         parentChain: _buildParentChain(metadata),
-        mediaType: metadata.type,
+        mediaType: metadata.type ?? '',
         viewOffset: viewOffset,
         isNowWatched: isNowWatched,
       ),