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/ChatView.vue

@@ -7,6 +7,7 @@ import { apiGet, transcribeAudio, synthesiseSpeech } from "@/api/client";
 import { renderMarkdown } from "@/utils/markdown";
 import { useVoiceRecorder } from "@/composables/useVoiceRecorder";
 import { useVoiceAudio, setVoiceVolume } from "@/composables/useVoiceAudio";
+import { useListenMode } from "@/composables/useListenMode";
 import ChatMessage from "@/components/ChatMessage.vue";
 import ToolCallCard from "@/components/ToolCallCard.vue";
 import ToolConfirmCard from "@/components/ToolConfirmCard.vue";
@@ -30,7 +31,7 @@ const recorder = useVoiceRecorder();
 const audio = useVoiceAudio();
 const voiceEnabled = computed(() => settingsStore.voiceSttReady);
 const voiceTtsEnabled = computed(() => settingsStore.voiceTtsReady);
-const listenMode = ref(false);
+const listenMode = useListenMode();
 const synthesising = ref(false);
 const showVolumeSlider = ref(false);