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/libraries/folder_tree_item.dart

@@ -42,21 +42,14 @@ class FolderTreeItem extends StatelessWidget {
     }
 
     // File icons based on type
-    final type = item.type.toLowerCase();
-    switch (type) {
-      case 'movie':
-        return Symbols.movie_rounded;
-      case 'show':
-        return Symbols.tv_rounded;
-      case 'season':
-        return Symbols.video_library_rounded;
-      case 'episode':
-        return Symbols.play_circle_rounded;
-      case 'collection':
-        return Symbols.collections_rounded;
-      default:
-        return Symbols.insert_drive_file_rounded;
-    }
+    return switch (item.mediaType) {
+      PlexMediaType.movie => Symbols.movie_rounded,
+      PlexMediaType.show => Symbols.tv_rounded,
+      PlexMediaType.season => Symbols.video_library_rounded,
+      PlexMediaType.episode => Symbols.play_circle_rounded,
+      PlexMediaType.collection => Symbols.collections_rounded,
+      _ => Symbols.insert_drive_file_rounded,
+    };
   }
 
   void _handleTap() {
@@ -104,7 +97,7 @@ class FolderTreeItem extends StatelessWidget {
           // Item title
           Expanded(
             child: Text(
-              item.title,
+              item.displayTitle,
               style: TextStyle(fontSize: 14, fontWeight: isFolder ? FontWeight.w500 : FontWeight.w400),
               maxLines: 1,
               overflow: TextOverflow.ellipsis,