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/App.vue

@@ -22,6 +22,7 @@ const { showShortcuts, toggleShortcuts, closeShortcuts } = useShortcuts();
 function startAppServices() {
   chatStore.startStatusPolling();
   settingsStore.fetchSettings();
+  settingsStore.checkVoiceStatus();
   // Sync browser timezone to the server on every login/page load.
   apiPut("/api/settings", { user_timezone: Intl.DateTimeFormat().resolvedOptions().timeZone }).catch(() => {});
 }