feat: listen mode + volume knob in chat; briefing discuss auto-send; fix LLM proactive note search

- ChatView: listen mode toggle (auto-reads new responses via TTS), volume popup
  with range slider persisted per-device in localStorage via GainNode
- useVoiceAudio: shared module-level _volume ref with localStorage persistence,
  GainNode for volume control, exported setVoiceVolume()
- tts.py: pre-warm all Kokoro voices at pipeline load to eliminate HuggingFace
  HEAD requests at synthesis time (reduces TTS latency)
- BriefingView: discuss article button now auto-sends instead of just filling input;
  prompt capped to 15 sentences; send() accepts optional overrideText
- llm.py: instruct LLM not to proactively search notes or comment on note absence

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

frontend/src/views/BriefingView.vue

@@ -160,8 +160,8 @@ watch(() => chatStore.streaming, async (streaming) => {
 const input = ref('')
 const sending = ref(false)
 
-async function send() {
-  const text = input.value.trim()
+async function send(overrideText?: string) {
+  const text = (overrideText ?? input.value).trim()
   if (!text || !todayConvId.value || chatStore.streaming || sending.value) return
   if (chatStore.currentConversation?.id !== todayConvId.value) {
     await chatStore.fetchConversation(todayConvId.value)
@@ -175,13 +175,14 @@ async function send() {
   }
 }
 
-function discussArticle(item: NewsItem) {
+async function discussArticle(item: NewsItem) {
   const snippet = item.snippet ? `\n\n"${item.snippet.slice(0, 400)}${item.snippet.length > 400 ? '…' : ''}"` : ''
-  input.value = `Can you summarize and discuss this article for me?\n\n**${item.title}**${snippet}\n\nSource: ${item.source}`
-  // Scroll the chat panel into view on narrow layouts
-  nextTick(() => {
+  const text = `Please give me a brief summary and key takeaways for this article (15 sentences or fewer):\n\n**${item.title}**${snippet}\n\nSource: ${item.source}`
+  // Scroll the chat panel into view on narrow layouts, then send
+  await nextTick(() => {
     document.querySelector('.briefing-chat')?.scrollIntoView({ behavior: 'smooth', block: 'nearest' })
   })
+  await send(text)
 }
 
 function onKeydown(e: KeyboardEvent) {
@@ -490,7 +491,7 @@ onMounted(async () => {
           </button>
           <button
             class="btn-send"
-            @click="send"
+            @click="send()"
             :disabled="!input.trim() || chatStore.streaming || sending"
             aria-label="Send"
           >&uarr;</button>