Fix mobile app: AccessToken for voice, Agent Manager for status, caller ID support

- Voice token: use AccessToken + VoiceGrant instead of browser-only ClientToken
- Agent status: delegate to TWP_Agent_Manager matching browser phone behavior
- Queue loading: add missing require_once for TWP_User_Queue_Manager
- Add /phone-numbers endpoint for caller ID selection
- Webhook: support CallerId param from mobile extraOptions
- Flutter: caller ID dropdown in dialer, error logging in all catch blocks

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

mobile/lib/screens/dashboard_screen.dart

@@ -76,7 +76,7 @@ class _DashboardScreenState extends State<DashboardScreen> {
                   if (phoneNumbers.isNotEmpty) ...[
                     const SizedBox(height: 12),
                     DropdownButtonFormField<String>(
-                      value: selectedCallerId,
+                      initialValue: selectedCallerId,
                       decoration: const InputDecoration(
                         labelText: 'Caller ID',
                         isDense: true,