fix: stop TTS playback when PTT is activated

Pressing push-to-talk now immediately stops any ongoing TTS audio before
opening the microphone, preventing the assistant from hearing itself.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/BriefingView.vue

@@ -281,6 +281,7 @@ async function speakText(text: string) {
 // PTT: hold to record, release to transcribe and send
 async function startPtt() {
   if (!voiceEnabled.value || recorder.recording.value) return
+  audio.stop()
   await recorder.startRecording()
 }