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/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: