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/components/DashboardChatInput.vue

@@ -1,8 +1,9 @@
 <script setup lang="ts">
-import { ref, nextTick, onMounted } from "vue";
-import { apiGet, getVoiceStatus, transcribeAudio } from "@/api/client";
+import { ref, computed, nextTick } from "vue";
+import { apiGet, transcribeAudio } from "@/api/client";
 import { useVoiceRecorder } from "@/composables/useVoiceRecorder";
 import { useChatStore } from "@/stores/chat";
+import { useSettingsStore } from "@/stores/settings";
 import type { Note } from "@/types/note";
 
 const emit = defineEmits<{
@@ -108,17 +109,11 @@ function focus() {
 }
 
 // Voice PTT
-const voiceEnabled = ref(false);
+const settingsStore = useSettingsStore();
+const voiceEnabled = computed(() => settingsStore.voiceSttReady);
 const transcribingVoice = ref(false);
 const recorder = useVoiceRecorder();
 
-onMounted(async () => {
-  try {
-    const status = await getVoiceStatus();
-    voiceEnabled.value = status.enabled && status.stt;
-  } catch { /* voice absent */ }
-});
-
 async function startPtt() {
   if (!voiceEnabled.value || recorder.recording.value) return;
   await recorder.startRecording();