Fix QThread lifecycle crash: 'Destroyed while thread is still running'

- Add _cleanup_worker() helper: calls wait() then deleteLater() before
  dropping the Python reference. This ensures the C++ thread is fully
  cleaned up before the QObject is destroyed.
- Replace all 'self.worker_thread = None' with _cleanup_worker()
- Add running thread guards to all handlers: prevent starting a new
  WorkerThread while one is already running
- Add timeout to wait() calls (3000ms) to avoid hanging on close
- WorkerThread.stop() now calls self.wait() to ensure clean shutdown

src/main.py

@@ -201,6 +201,7 @@ class WorkerThread(QThread):
 
     def stop(self):
         self.is_running = False
+        self.wait()
 
 
 class MainWindow(QMainWindow):
@@ -222,6 +223,18 @@ class MainWindow(QMainWindow):
         self._scan_library()
         self._on_refresh_devices_clicked()
 
+    def _cleanup_worker(self):
+        """Safely clean up the worker thread after it finishes.
+        
+        Calls wait() to ensure the C++ thread is fully cleaned up before
+        dropping the Python reference. This prevents the Qt crash:
+        'QThread: Destroyed while thread is still running'
+        """
+        if self.worker_thread:
+            self.worker_thread.wait(3000)
+            self.worker_thread.deleteLater()
+            self.worker_thread = None
+
     def _setup_ui(self):
         main_widget = QWidget()
         main_layout = QVBoxLayout(main_widget)
@@ -632,6 +645,10 @@ class MainWindow(QMainWindow):
             QMessageBox.warning(self, "Error", "Please enter a valid URL")
             return
 
+        if self.worker_thread and self.worker_thread.isRunning():
+            QMessageBox.information(self, "Info", "Please wait for the current task to finish")
+            return
+
         self.download_button.setEnabled(False)
         self.download_status.setText("Downloading...")
         self.download_progress.setValue(0)
@@ -669,7 +686,7 @@ class MainWindow(QMainWindow):
             self.downloaded_tracks = []
             self.track_list.clear()
 
-        self.worker_thread = None
+        self._cleanup_worker()
 
     def _on_library_convert_clicked(self):
         source_files = self._get_selected_source_files()
@@ -682,6 +699,10 @@ class MainWindow(QMainWindow):
             )
             return
 
+        if self.worker_thread and self.worker_thread.isRunning():
+            QMessageBox.information(self, "Info", "Please wait for the current task to finish")
+            return
+
         output_dir = self.output_dir_input.text().strip()
         if not output_dir:
             QMessageBox.warning(self, "Error", "Please set an output directory")
@@ -718,7 +739,7 @@ class MainWindow(QMainWindow):
             self.library_status.setText(f"Error: {message}")
             QMessageBox.warning(self, "Conversion Error", message)
 
-        self.worker_thread = None
+        self._cleanup_worker()
 
     def _on_library_transfer_clicked(self):
         selected_tracks = self._get_selected_ready_tracks()
@@ -730,6 +751,10 @@ class MainWindow(QMainWindow):
             QMessageBox.warning(self, "Error", "No iPod device mounted. Go to iPod tab and mount first.")
             return
 
+        if self.worker_thread and self.worker_thread.isRunning():
+            QMessageBox.information(self, "Info", "Please wait for the current task to finish")
+            return
+
         self.library_transfer_btn.setEnabled(False)
         self.library_status.setText("Transferring...")
         self.library_progress.setValue(0)
@@ -759,9 +784,12 @@ class MainWindow(QMainWindow):
             self.library_status.setText(f"Error: {message}")
             QMessageBox.warning(self, "Transfer Error", message)
 
-        self.worker_thread = None
+        self._cleanup_worker()
 
     def _on_refresh_devices_clicked(self):
+        if self.worker_thread and self.worker_thread.isRunning():
+            return
+
         self.device_combo.clear()
         self.device_info_label.setText("Detecting devices...")
 
@@ -807,7 +835,7 @@ class MainWindow(QMainWindow):
             self._set_device_not_found()
             self.statusBar().showMessage(message)
 
-        self.worker_thread = None
+        self._cleanup_worker()
 
     def _set_device_mounted(self, device: dict, mount_point: str):
         self.current_mount_point = mount_point
@@ -1010,7 +1038,8 @@ class MainWindow(QMainWindow):
     def closeEvent(self, event):
         if self.worker_thread and self.worker_thread.isRunning():
             self.worker_thread.stop()
-            self.worker_thread.wait()
+            self.worker_thread.wait(5000)
+            self.worker_thread.deleteLater()
         event.accept()