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/SettingsView.vue

@@ -484,6 +484,8 @@ async function reloadVoiceModels() {
         if (status.stt && status.tts) {
           voiceStatus.value = status;
           voiceTabLoaded.value = true;
+          // Propagate to global store so mic buttons appear everywhere
+          store.checkVoiceStatus();
           break;
         }
       } catch { /* keep polling */ }
@@ -569,6 +571,28 @@ async function loadVoiceTab() {
   }
 }
 
+const voicePreviewing = ref(false);
+let _voicePreviewUrl = "";
+
+async function previewVoice() {
+  if (voicePreviewing.value) return;
+  voicePreviewing.value = true;
+  try {
+    const blob = await synthesiseSpeech(
+      "Hello! I'm your assistant. How can I help you today?",
+      voiceTtsVoice.value,
+      voiceTtsSpeed.value,
+    );
+    if (_voicePreviewUrl) URL.revokeObjectURL(_voicePreviewUrl);
+    _voicePreviewUrl = URL.createObjectURL(blob);
+    new Audio(_voicePreviewUrl).play();
+  } catch {
+    toastStore.show("Preview failed — TTS may not be ready", "error");
+  } finally {
+    voicePreviewing.value = false;
+  }
+}
+
 async function saveVoiceSettings() {
   savingVoice.value = true;
   voiceSaved.value = false;
@@ -2250,6 +2274,9 @@ function formatUserDate(iso: string): string {
         </div>
 
         <div class="form-actions">
+          <button class="btn-secondary" @click="previewVoice" :disabled="voicePreviewing || !voiceStatus?.tts">
+            {{ voicePreviewing ? 'Generating…' : '▶ Preview' }}
+          </button>
           <button class="btn-save" @click="saveVoiceSettings" :disabled="savingVoice">
             {{ voiceSaved ? 'Saved ✓' : savingVoice ? 'Saving…' : 'Save Voice Settings' }}
           </button>