fix: harden AI design voice lifecycle

src/pages/ImageCanvas/index.tsx

@@ -1426,6 +1426,7 @@ export function ImageCanvas() {
         for (const track of stream.getTracks()) track.stop();
         return;
       }
+      voiceStreamRef.current = stream;
       const preferredMimeType = window.MediaRecorder.isTypeSupported?.('audio/webm')
         ? 'audio/webm'
         : '';
@@ -1433,7 +1434,6 @@ export function ImageCanvas() {
         stream,
         preferredMimeType ? { mimeType: preferredMimeType } : undefined,
       );
-      voiceStreamRef.current = stream;
       voiceChunksRef.current = [];
       voiceRecorderRef.current = recorder;
       recorder.ondataavailable = (event) => {