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/models/plex_metadata.dart

@@ -62,11 +62,11 @@ enum PlexMediaType {
 class PlexMetadata with MultiServerFields {
   @JsonKey(readValue: _readRatingKey)
   final String ratingKey;
-  final String key;
+  final String? key;
   final String? guid;
   final String? studio;
-  final String type;
-  final String title;
+  final String? type;
+  final String? title;
   final String? titleSort;
   final String? contentRating;
   final String? summary;
@@ -135,7 +135,8 @@ class PlexMetadata with MultiServerFields {
 
   /// Parsed media type enum for type-safe comparisons
   PlexMediaType get mediaType {
-    return switch (type.toLowerCase()) {
+    if (type == null) return PlexMediaType.unknown;
+    return switch (type!.toLowerCase()) {
       'movie' => PlexMediaType.movie,
       'show' => PlexMediaType.show,
       'season' => PlexMediaType.season,
@@ -153,11 +154,11 @@ class PlexMetadata with MultiServerFields {
 
   PlexMetadata({
     required this.ratingKey,
-    required this.key,
+    this.key,
     this.guid,
     this.studio,
-    required this.type,
-    required this.title,
+    this.type,
+    this.title,
     this.titleSort,
     this.contentRating,
     this.summary,
@@ -366,7 +367,7 @@ class PlexMetadata with MultiServerFields {
 
   // Helper to get the display title (show name for episodes/seasons, title otherwise)
   String get displayTitle {
-    final itemType = type.toLowerCase();
+    final itemType = type?.toLowerCase();
 
     // For episodes and seasons, prefer grandparent title (show name)
     if ((itemType == 'episode' || itemType == 'season') && grandparentTitle != null) {
@@ -376,12 +377,12 @@ class PlexMetadata with MultiServerFields {
     if (itemType == 'season' && parentTitle != null) {
       return parentTitle!;
     }
-    return title;
+    return title ?? '';
   }
 
   // Helper to get the subtitle (episode/season title)
   String? get displaySubtitle {
-    final itemType = type.toLowerCase();
+    final itemType = type?.toLowerCase();
 
     if (itemType == 'episode' || itemType == 'season') {
       // If we showed grandparent/parent as title, show this item's title as subtitle
@@ -401,7 +402,7 @@ class PlexMetadata with MultiServerFields {
   /// For movies/shows/seasons in mixed hub context: returns art (16:9 background)
   /// For other types: returns thumb
   String? posterThumb({EpisodePosterMode mode = EpisodePosterMode.seriesPoster, bool mixedHubContext = false}) {
-    final itemType = type.toLowerCase();
+    final itemType = type?.toLowerCase();
 
     if (itemType == 'episode') {
       switch (mode) {
@@ -436,7 +437,7 @@ class PlexMetadata with MultiServerFields {
   /// Clips (trailers, extras) always use 16:9.
   /// Movies, shows, and seasons use 16:9 in mixed hub context with episodeThumbnail mode.
   bool usesWideAspectRatio(EpisodePosterMode mode, {bool mixedHubContext = false}) {
-    final itemType = type.toLowerCase();
+    final itemType = type?.toLowerCase();
     // Clips (trailers, extras) are always 16:9
     if (itemType == 'clip') return true;
     if (itemType == 'episode' && mode == EpisodePosterMode.episodeThumbnail) {

lib/models/plex_metadata.g.dart

@@ -8,11 +8,11 @@ part of 'plex_metadata.dart';
 
 PlexMetadata _$PlexMetadataFromJson(Map<String, dynamic> json) => PlexMetadata(
   ratingKey: _readRatingKey(json, 'ratingKey') as String,
-  key: json['key'] as String,
+  key: json['key'] as String?,
   guid: json['guid'] as String?,
   studio: json['studio'] as String?,
-  type: json['type'] as String,
-  title: json['title'] as String,
+  type: json['type'] as String?,
+  title: json['title'] as String?,
   titleSort: json['titleSort'] as String?,
   contentRating: json['contentRating'] as String?,
   summary: json['summary'] as String?,