Fix CFNetwork SIGABRT crash when creating download tasks on invalidated session

The background URLSession could be in an invalid state when downloadTask(with:)
is called, because invalidateAndCancel() is asynchronous internally. This adds
an ObjC exception handler to catch NSExceptions from CFNetwork, nil guards on
the session, and safer session lifecycle management (nil after invalidation,
finishTasksAndInvalidate for cellular toggle).

Yattee/Services/Downloads/DownloadManager.swift

@@ -148,8 +148,9 @@ final class DownloadManager: NSObject {
         self.downloadSettings = settings
         // Invalidate old session before creating new one with correct settings
         urlSession?.invalidateAndCancel()
+        urlSession = nil
         setupSession()
-        
+
         // Resume interrupted downloads only on initial setup
         if isInitialSetup {
             resumeInterruptedDownloads()
@@ -186,8 +187,9 @@ final class DownloadManager: NSObject {
             }
         }
 
-        // 3. Invalidate the old session
-        urlSession.invalidateAndCancel()
+        // 3. Invalidate the old session (use finishTasksAndInvalidate since downloads are already paused)
+        urlSession?.finishTasksAndInvalidate()
+        urlSession = nil
 
         // 4. Create new session with updated cellular config
         setupSession()