Assistant: block "Nueva conversación" while a run is streaming

MAF persists the thread at run END, so clearing mid-stream deleted the
row only for the finishing run to write the conversation right back —
the thread reappeared after the reload ("chunks" needing a second
clear). The button is now disabled while agent.isRunning.

(The chunked-clear reports on the prior build had a second cause fixed
in 0f34a64: clearing left the client's message copy alive and the next
question re-persisted it.)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/Asistente.tsx

@@ -144,7 +144,10 @@ export default function Asistente() {
               variant="outline"
               size="sm"
               onClick={() => setConfirmClear(true)}
-              disabled={!agent || agent.messages.length === 0}
+              // Also disabled mid-run: MAF saves the thread at run END, so
+              // clearing while an answer streams would delete the row only
+              // to have the finishing run write it right back.
+              disabled={!agent || agent.messages.length === 0 || agent.isRunning}
             >
               <MessageSquarePlus />
               Nueva conversación