fix: no text displayed during mic capture

DashScope realtime ASR sends utterance-completed (final) events
without incremental deltas. The onmessage handler cleared
partialTranscript on every final, so text never appeared.

Set partialTranscript to full_text on final messages instead
of clearing it, keeping the transcript visible in QueryInput.

frontend/src/hooks/useMediaStreamASR.ts

@@ -142,7 +142,7 @@ export function useMediaStreamASR({ wsUrl }: UseMediaStreamASRProps): UseMediaSt
           transcriptRef.current = msg.full_text
           lastStashRef.current = ''
           setTranscript(msg.full_text)
-          setPartialTranscript('')
+          setPartialTranscript(msg.full_text)
         } else if (msg.delta) {
           transcriptRef.current += msg.delta
           lastStashRef.current = (msg as any).stash || ''