feat(voice): replace silence detector with Silero VAD in ChatInputBar

Swaps useSilenceDetector for useVad. Adds no-speech guard: when the
user manually stops recording without VAD ever detecting speech, show
a toast and skip the Whisper round-trip instead of sending pure noise.

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

frontend/src/components/ChatInputBar.vue

@@ -1,8 +1,8 @@
 <script setup lang="ts">
-import { ref, computed, nextTick, onMounted, onUnmounted } from 'vue'
+import { ref, computed, nextTick, onMounted, onUnmounted, watch } from 'vue'
 import { apiGet, transcribeAudio } from '@/api/client'
 import { useVoiceRecorder } from '@/composables/useVoiceRecorder'
-import { useSilenceDetector } from '@/composables/useSilenceDetector'
+import { useVad } from '@/composables/useVad'
 import { useStreamingTts } from '@/composables/useStreamingTts'
 import { useVoiceAudio, setVoiceVolume } from '@/composables/useVoiceAudio'
 import { useListenMode } from '@/composables/useListenMode'
@@ -138,29 +138,49 @@ function removeAttachedNote() {
   attachedNote.value = null
 }
 
-// ── Voice (click-to-toggle + silence detection) ─────────────────────────────
+// ── Voice (click-to-toggle + VAD speech detection) ─────────────────────────
 const transcribingVoice = ref(false)
 const recorder = useVoiceRecorder()
-const silenceDetector = useSilenceDetector()
+// Tracks whether VAD detected speech during the current recording session.
+// Used to skip the Whisper round-trip when the user manually stops without
+// ever speaking — the recording is ambient noise and will transcribe to "".
+const vadSawSpeech = ref(false)
+
+const vad = useVad({
+  onSpeechEnd: () => {
+    // VAD auto-stop: speech was detected and the user has paused. Proceed
+    // to transcribe the MediaRecorder output.
+    void stopRecording(false)
+  },
+  onNoSpeech: () => {
+    // User manually stopped without VAD detecting speech. Toast it; the
+    // stopRecording path below will skip Whisper based on vadSawSpeech.
+    useToastStore().show('No speech detected', 'warning')
+  },
+})
 
 // Live mic halo. A solid red disc sits behind the mic button and scales
-// with `silenceDetector.amplitude` (0..1 RMS, already amplified for
-// visibility). The button itself stays put so the mic icon is always
-// legible on top. 0.2 baseline breathes on silence; loud speech drives
-// the disc out to ~2.6× the button size with a softer radial fade.
+// with `vad.amplitude` (0..1 RMS, already amplified for visibility).
 const micGlowStyle = computed(() => {
   if (!recorder.recording.value) return { display: 'none' }
-  const pulse = 0.2 + Math.min(silenceDetector.amplitude.value, 1) * 0.8
+  const pulse = 0.2 + Math.min(vad.amplitude.value, 1) * 0.8
   return {
     transform: `translate(-50%, -50%) scale(${1 + pulse * 1.6})`,
     opacity: String(0.45 + pulse * 0.4),
   }
 })
 
+// vad.speaking flips true on speech-start. Watch it once per session to
+// capture that speech was detected at some point, without clearing when
+// speech ends. This drives the no-speech guard in stopRecording.
+watch(() => vad.speaking.value, (on) => {
+  if (on) vadSawSpeech.value = true
+})
+
 async function toggleVoice() {
   if (transcribingVoice.value) return
   if (recorder.recording.value) {
-    await stopRecording()
+    await stopRecording(true)
   } else {
     await startRecording()
   }
@@ -172,22 +192,32 @@ async function startRecording() {
     useToastStore().show('Microphone requires HTTPS or localhost', 'error')
     return
   }
+  vadSawSpeech.value = false
   await recorder.startRecording()
   if (recorder.error.value) {
     useToastStore().show(recorder.error.value, 'error')
     return
   }
   if (recorder.stream.value) {
-    silenceDetector.start(recorder.stream.value, () => stopRecording())
+    await vad.start(recorder.stream.value)
   }
 }
 
-async function stopRecording() {
-  silenceDetector.stop()
+async function stopRecording(manual: boolean) {
+  if (manual) {
+    await vad.stopAndCheck()
+  } else {
+    await vad.stop()
+  }
   if (!recorder.recording.value) return
   transcribingVoice.value = true
   try {
     const blob = await recorder.stopRecording()
+    // No-speech guard: user manually stopped without VAD seeing speech.
+    // Skip Whisper — the toast was already shown by onNoSpeech.
+    if (manual && !vadSawSpeech.value) {
+      return
+    }
     // Pass last assistant message as context to reduce STT mishearings
     const msgs = store.currentConversation?.messages ?? []
     const lastAssistant = [...msgs].reverse().find(m => m.role === 'assistant')?.content