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/composables/useVoiceAudio.ts

@@ -1,12 +1,10 @@
 import { ref, readonly } from 'vue'
 
-/**
- * Audio playback composable wrapping the Web Audio API.
- *
- * Usage:
- *   const { playing, isSupported, play, stop } = useVoiceAudio()
- *   await play(wavBlob)
- */
+const VOLUME_KEY = 'fa_voice_volume'
+
+// Shared volume across all instances — persisted to localStorage per device
+const _volume = ref<number>(parseFloat(localStorage.getItem(VOLUME_KEY) ?? '1.0'))
+
 export function useVoiceAudio() {
   const playing = ref(false)
   const isSupported = typeof AudioContext !== 'undefined' || typeof (window as unknown as Record<string, unknown>).webkitAudioContext !== 'undefined'
@@ -32,7 +30,12 @@ export function useVoiceAudio() {
 
     const source = ctx.createBufferSource()
     source.buffer = audioBuffer
-    source.connect(ctx.destination)
+
+    const gain = ctx.createGain()
+    gain.gain.value = _volume.value
+    source.connect(gain)
+    gain.connect(ctx.destination)
+
     currentSource = source
     playing.value = true
 
@@ -53,8 +56,14 @@ export function useVoiceAudio() {
 
   return {
     playing: readonly(playing),
+    volume: _volume,
     isSupported,
     play,
     stop,
   }
 }
+
+export function setVoiceVolume(v: number) {
+  _volume.value = Math.max(0, Math.min(1, v))
+  localStorage.setItem(VOLUME_KEY, String(_volume.value))
+}