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>

client/transcription_engine.py

@@ -221,9 +221,16 @@ class TranscriptionEngine:
     def unload_model(self):
         """Unload the model from memory."""
         with self.model_lock:
+            if self.model is not None:
+                # Delete the model reference
+                del self.model
             self.model = None
             self.is_loaded = False
 
+        # Force garbage collection to free memory
+        import gc
+        gc.collect()
+
     def __repr__(self) -> str:
         return f"TranscriptionEngine(model={self.model_size}, device={self.device}, loaded={self.is_loaded})"