fix(desktop): preserve active chat when switching models (#648)

Switching models from the command palette called createConversation() on every change, creating a persisted empty "New chat" entry and pulling the user out of their active conversation. Because updateLastAssistant writes the visible messages array without checking the active conversation, a mid-stream switch could also clobber the new chat's view with the old conversation's messages.

Remove the conversation-creation side effect. Model switching now preserves the active chat (matching the pull-completion and delete-fallback paths, which already switched silently); the next request uses the newly selected model with the current conversation context. Preloading, loading state, and logging are unchanged.

frontend/src/components/CommandPalette.tsx

@@ -149,8 +149,7 @@ export function CommandPalette() {
     setCommandPaletteOpen(false);
 
     if (modelId !== previousModel) {
-      const { createConversation, setModelLoading, addLogEntry } = useAppStore.getState();
-      createConversation(modelId);
+      const { setModelLoading, addLogEntry } = useAppStore.getState();
       setModelLoading(true);
       addLogEntry({ timestamp: Date.now(), level: 'info', category: 'model', message: `Switching to ${modelId}...` });
       try {