Fix model switching crash and improve error handling

**Model Reload Fixes:**
- Properly disconnect signals before reconnecting to prevent duplicate connections
- Wait for previous model loader thread to finish before starting new one
- Add garbage collection after unloading model to free memory
- Improve error handling in model reload callback

**Settings Dialog:**
- Remove duplicate success message (callback handles it)
- Only show message if no callback is defined

**Transcription Engine:**
- Explicitly delete model reference before setting to None
- Force garbage collection to ensure memory is freed

This prevents crashes when switching models, especially when done
multiple times in succession or while the app is under load.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

gui/main_window_qt.py

@@ -571,6 +571,18 @@ class MainWindow(QMainWindow):
             self.status_label.setText("⚙ Reloading model...")
             self.start_button.setEnabled(False)
 
+            # Wait for any existing model loader thread to finish and disconnect
+            if self.model_loader_thread and self.model_loader_thread.isRunning():
+                print("Waiting for previous model loader to finish...")
+                self.model_loader_thread.wait()
+
+            # Disconnect any existing signals to prevent duplicate connections
+            if self.model_loader_thread:
+                try:
+                    self.model_loader_thread.finished.disconnect()
+                except:
+                    pass  # Already disconnected or never connected
+
             # Unload current model
             if self.transcription_engine:
                 try:
@@ -600,10 +612,7 @@ class MainWindow(QMainWindow):
                 min_confidence=self.config.get('processing.min_confidence', 0.5)
             )
 
-            # Load model in background thread
-            if self.model_loader_thread and self.model_loader_thread.isRunning():
-                self.model_loader_thread.wait()
-
+            # Create new model loader thread
             self.model_loader_thread = ModelLoaderThread(self.transcription_engine)
             self.model_loader_thread.finished.connect(self._on_model_reloaded)
             self.model_loader_thread.start()
@@ -619,23 +628,28 @@ class MainWindow(QMainWindow):
 
     def _on_model_reloaded(self, success: bool, message: str):
         """Handle model reloading completion."""
-        if success:
-            # Update device label with actual device used
-            if self.transcription_engine:
-                actual_device = self.transcription_engine.device
-                compute_type = self.transcription_engine.compute_type
-                device_display = f"{actual_device.upper()} ({compute_type})"
-                self.device_label.setText(f"Device: {device_display}")
+        try:
+            if success:
+                # Update device label with actual device used
+                if self.transcription_engine:
+                    actual_device = self.transcription_engine.device
+                    compute_type = self.transcription_engine.compute_type
+                    device_display = f"{actual_device.upper()} ({compute_type})"
+                    self.device_label.setText(f"Device: {device_display}")
 
-            host = self.config.get('web_server.host', '127.0.0.1')
-            port = self.config.get('web_server.port', 8080)
-            self.status_label.setText(f"✓ Ready | Web: http://{host}:{port}")
-            self.start_button.setEnabled(True)
-            QMessageBox.information(self, "Settings Saved", "Model reloaded successfully with new settings!")
-        else:
-            self.status_label.setText("❌ Model loading failed")
-            QMessageBox.critical(self, "Error", f"Failed to reload model:\n{message}")
-            self.start_button.setEnabled(False)
+                host = self.config.get('web_server.host', '127.0.0.1')
+                port = self.config.get('web_server.port', 8080)
+                self.status_label.setText(f"✓ Ready | Web: http://{host}:{port}")
+                self.start_button.setEnabled(True)
+                QMessageBox.information(self, "Settings Saved", "Model reloaded successfully with new settings!")
+            else:
+                self.status_label.setText("❌ Model loading failed")
+                QMessageBox.critical(self, "Error", f"Failed to reload model:\n{message}")
+                self.start_button.setEnabled(False)
+        except Exception as e:
+            print(f"Error in _on_model_reloaded: {e}")
+            import traceback
+            traceback.print_exc()
 
     def _start_server_sync(self):
         """Start server sync client."""

gui/settings_dialog_qt.py

@@ -283,11 +283,13 @@ class SettingsDialog(QDialog):
             self.config.set('server_sync.room', self.server_room_input.text())
             self.config.set('server_sync.passphrase', self.server_passphrase_input.text())
 
-            # Call save callback
+            # Call save callback (which will show the success message)
             if self.on_save:
                 self.on_save()
+            else:
+                # Only show message if no callback
+                QMessageBox.information(self, "Settings Saved", "Settings have been saved successfully!")
 
-            QMessageBox.information(self, "Settings Saved", "Settings have been saved successfully!")
             self.accept()
 
         except ValueError as e: