fix: voice status global store, per-view mic reactivity, single-voice preview

- Move voice status into settings store (voiceSttReady, voiceTtsReady),
  checked once at login and refreshed after admin model reload
- ChatView, BriefingView, DashboardChatInput now use computed refs from
  the store — mic buttons appear reactively without needing a page reload
- BriefingView: separate STT-only guard for mic PTT vs TTS-only guard for
  listen mode / speak buttons
- Add ▶ Preview button to Voice & Speed section in Settings for single-
  voice testing without enabling blend mode

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

frontend/src/views/ChatView.vue

@@ -3,7 +3,7 @@ import { onMounted, onUnmounted, ref, computed, watch, nextTick } from "vue";
 import { useRoute, useRouter } from "vue-router";
 import { useChatStore } from "@/stores/chat";
 import { useSettingsStore } from "@/stores/settings";
-import { apiGet, getVoiceStatus, transcribeAudio } from "@/api/client";
+import { apiGet, transcribeAudio } from "@/api/client";
 import { renderMarkdown } from "@/utils/markdown";
 import { useVoiceRecorder } from "@/composables/useVoiceRecorder";
 import ChatMessage from "@/components/ChatMessage.vue";
@@ -23,17 +23,10 @@ const sending = ref(false);
 const summarizing = ref(false);
 const sidebarOpen = ref(false);
 
-// Voice PTT
-const voiceEnabled = ref(false);
+// Voice PTT — use store so status is globally reactive
 const transcribingVoice = ref(false);
 const recorder = useVoiceRecorder();
-
-async function checkVoice() {
-  try {
-    const status = await getVoiceStatus();
-    voiceEnabled.value = status.enabled && status.stt;
-  } catch { /* voice absent */ }
-}
+const voiceEnabled = computed(() => settingsStore.voiceSttReady);
 
 async function startPtt() {
   if (!voiceEnabled.value || recorder.recording.value) return;
@@ -178,7 +171,7 @@ const inputPlaceholder = computed(() => {
 
 onMounted(async () => {
   document.addEventListener("keydown", onGlobalKeydown);
-  await Promise.all([store.fetchConversations(), loadProjects(), checkVoice()]);
+  await Promise.all([store.fetchConversations(), loadProjects()]);
   if (convId.value) {
     if (store.currentConversation?.id !== convId.value) {
       await store.fetchConversation(convId.value);