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/screens/playlist/playlist_item_card.dart

@@ -237,9 +237,9 @@ class _PlaylistItemCardState extends State<PlaylistItemCard> {
   }
 
   String _buildSubtitle() {
-    final itemType = widget.item.type.toLowerCase();
+    final itemType = widget.item.mediaType;
 
-    if (itemType == 'episode') {
+    if (itemType == PlexMediaType.episode) {
       // For episodes, show "S#E# - Episode Title"
       final season = widget.item.parentIndex;
       final episode = widget.item.index;
@@ -247,7 +247,7 @@ class _PlaylistItemCardState extends State<PlaylistItemCard> {
         return 'S${season}E$episode${widget.item.displaySubtitle != null ? ' - ${widget.item.displaySubtitle}' : ''}';
       }
       return widget.item.displaySubtitle ?? t.discover.tvShow;
-    } else if (itemType == 'movie') {
+    } else if (itemType == PlexMediaType.movie) {
       // For movies, show year and edition
       final year = widget.item.year?.toString();
       if (year != null && widget.item.editionTitle != null) {
@@ -257,6 +257,6 @@ class _PlaylistItemCardState extends State<PlaylistItemCard> {
     }
 
     // Default to type
-    return widget.item.type;
+    return widget.item.mediaType.name;
   }
 }