feat: Migrate voice capture to composable with floating push-to-talk

Extract voice recording logic from FloatingVoice.vue into useVoiceCapture
composable. TranscriptCard now does real recording instead of mock typing.
InputSettings allows voice mode toggle (WebSpeech/Whisper GPU), mic
selection, and debug audio playback. ChatInput gets a settings gear button.

Long-press on FloatBubble shows a floating TranscriptCard (push-to-talk)
instead of opening the full PromptBar. Release stops recording after a
500ms buffer. Click still opens PromptBar normally.

Parallel MediaRecorder captures raw audio in WebSpeech mode for DB save
and debug playback. Transient errors (no-speech) no longer kill sessions.
Touch selection prevention on FloatBubble for tablets.

frontend/src/components/agent/TranscriptCard.vue

@@ -20,8 +20,10 @@ function handleStop() {
 }
 
 onMounted(() => {
-  props.voice.clearTranscript()
-  props.voice.startRecording()
+  if (!props.voice.isRecording.value) {
+    props.voice.clearTranscript()
+    props.voice.startRecording()
+  }
 })
 
 onBeforeUnmount(() => {