fix(chat): do not delete the answer when regenerating without a selected model

regenerateResponse removed the previous answer before checking for a model and then returned silently, leaving the conversation without a reply. It now checks first and explains that a model is required, like sendMessage. The duplicated streaming code in sendMessage and regenerateResponse is now a single streamAnswer helper.

__tests__/ChatScreen.test.tsx

@@ -165,7 +165,7 @@ describe('ChatScreen', () => {
             expect(apiMessages[0].role).toBe('system');
         });
 
-        it('edits without regenerating when no model is selected', () => {
+        it('edits and explains that a model is needed when none is selected', () => {
             const id = setup();
             act(() => {
                 useAppStore.setState({ activeModelId: '' });
@@ -177,8 +177,9 @@ describe('ChatScreen', () => {
                 jest.advanceTimersByTime(100);
             });
 
-            expect(state().messages).toHaveLength(1);
+            expect(state().messages).toHaveLength(2);
             expect(state().messages[0]).toMatchObject({ id, content: 'new question' });
+            expect(state().messages[1].content).toContain('No model selected');
             expect(streamMock).not.toHaveBeenCalled();
         });