Add nil safety checks for stream resolution handling

Added comprehensive nil checks for stream resolution values across PlayerBackend, QualityProfile, and PlayerQueue to prevent crashes when streams have missing resolution metadata. Also added backend nil checks in PlayerQueue.

Model/Player/PlayerQueue.swift

@@ -52,7 +52,7 @@ extension PlayerModel {
     func playItem(_ item: PlayerQueueItem, at time: CMTime? = nil) {
         advancing = false
 
-        if !playingInPictureInPicture, !currentItem.isNil {
+        if !playingInPictureInPicture, !currentItem.isNil, backend != nil {
             backend.closeItem()
         }
 
@@ -125,6 +125,12 @@ extension PlayerModel {
     }
 
     var streamByQualityProfile: Stream? {
+        // Safety check: Ensure backend is available
+        guard backend != nil else {
+            logger.error("Backend is nil when trying to select stream by quality profile")
+            return nil
+        }
+        
         let profile = qualityProfile ?? .defaultProfile
 
         // First attempt: Filter by both `canPlay` and `isPreferred`
@@ -229,7 +235,9 @@ extension PlayerModel {
             self.removeQueueItems()
         }
 
-        backend.closeItem()
+        if backend != nil {
+            backend.closeItem()
+        }
     }
 
     @discardableResult func enqueueVideo(