Use stable synthetic loopback prompt

app.js

@@ -115,6 +115,7 @@ const MIN_VAD_SILENCE_MS = 200;
 const MAX_VAD_SILENCE_MS = 800;
 const DEFAULT_LOOPBACK_SPEED = 1.0;
 const DEFAULT_LOOPBACK_PREROLL_MS = 180;
+const DEFAULT_LOOPBACK_PROMPT = "What application is this?";
 const MIC_START_TIMEOUT_MS = 15000;
 const WEBGPU_ADAPTER_TIMEOUT_MS = 5000;
 const DEFAULT_TTS_CHUNKING = Object.freeze({
@@ -2092,7 +2093,7 @@ function runLoopbackBenchmark(options = {}) {
   state.lastVadCloseAt = null;
   state.lastVadCloseDelayMs = null;
   state.loopbackRequestId += 1;
-  const text = APP_IDENTITY_PROMPT;
+  const text = typeof options?.text === "string" && options.text.trim() ? options.text.trim() : DEFAULT_LOOPBACK_PROMPT;
   startBenchmark("loopback", text);
   elements.partialTranscript.textContent = "Synthesizing local loopback speech...";
   elements.finalTranscript.textContent = "";