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/stores/settings.ts

@@ -1,6 +1,6 @@
 import { ref, computed } from "vue";
 import { defineStore } from "pinia";
-import { apiGet, apiPut } from "@/api/client";
+import { apiGet, apiPut, getVoiceStatus } from "@/api/client";
 import { useToastStore } from "@/stores/toast";
 import type { AppSettings } from "@/types/settings";
 
@@ -16,6 +16,24 @@ export const useSettingsStore = defineStore("settings", () => {
     () => settings.value.default_model || ""
   );
 
+  // Voice status — checked once on login, refreshable from Settings
+  const voiceEnabled = ref(false);
+  const voiceSttReady = ref(false);
+  const voiceTtsReady = ref(false);
+
+  async function checkVoiceStatus() {
+    try {
+      const s = await getVoiceStatus();
+      voiceEnabled.value = s.enabled;
+      voiceSttReady.value = s.enabled && s.stt;
+      voiceTtsReady.value = s.enabled && s.tts;
+    } catch {
+      voiceEnabled.value = false;
+      voiceSttReady.value = false;
+      voiceTtsReady.value = false;
+    }
+  }
+
   async function fetchSettings() {
     loading.value = true;
     try {
@@ -44,6 +62,10 @@ export const useSettingsStore = defineStore("settings", () => {
     loading,
     assistantName,
     defaultModel,
+    voiceEnabled,
+    voiceSttReady,
+    voiceTtsReady,
+    checkVoiceStatus,
     fetchSettings,
     updateSettings,
   };