refactor(chat): split ChatScreen and reuse regenerateResponse after editing a message

The edit path duplicated the streaming flow with inline require() calls; it now reuses regenerateResponse, which also fixes:
- edited conversations were never saved to History
- the answer generated after an edit could not be stopped (no abort handle kept)
Stopping a stream now saves the partial conversation, like finishing or failing does.
Adds characterization tests for ChatScreen and useLMStudio.

hooks/useLMStudio.ts

@@ -164,6 +164,7 @@ export function useLMStudio() {
             abortRef.current.abort();
             abortRef.current = null;
             setIsStreaming(false);
+            useAppStore.getState().saveCurrentChat();
         }
     }, [setIsStreaming]);