feat: persist listen mode across views via localStorage

Extract listen mode into a shared useListenMode() composable backed by
localStorage ('fa_listen_mode'). ChatView and BriefingView both use it,
so toggling auto-read on in one view keeps it on after navigation or
page refresh — no need to re-enable it each visit.

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

frontend/src/views/BriefingView.vue

@@ -3,6 +3,7 @@ import { ref, computed, onMounted, watch, nextTick } from 'vue'
 import { useBackgroundRefresh } from '@/composables/useBackgroundRefresh'
 import { useVoiceRecorder } from '@/composables/useVoiceRecorder'
 import { useVoiceAudio } from '@/composables/useVoiceAudio'
+import { useListenMode } from '@/composables/useListenMode'
 import { useChatStore } from '@/stores/chat'
 import { useSettingsStore } from '@/stores/settings'
 import ChatMessage from '@/components/ChatMessage.vue'
@@ -258,7 +259,7 @@ function convLabel(c: BriefingConversation): string {
 // Mic PTT needs STT; listen/speak mode also needs TTS
 const voiceEnabled = computed(() => settingsStore.voiceSttReady)
 const voiceTtsEnabled = computed(() => settingsStore.voiceTtsReady)
-const listenMode = ref(false)
+const listenMode = useListenMode()
 const transcribing = ref(false)
 const synthesising = ref(false)