fix: implement speaker/earpiece toggle on voice call page

Use Hardware.instance.setSpeakerphoneOn() to switch between speaker
and earpiece modes. Default to speaker on.

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

it0_app/lib/features/agent_call/presentation/pages/agent_call_page.dart

@@ -37,8 +37,9 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
   final List<double> _waveHeights = List.generate(20, (_) => 0.3);
   Timer? _waveTimer;
 
-  // Mute state
+  // Mute & speaker state
   bool _isMuted = false;
+  bool _isSpeaker = true;
 
   // Prevent double-actions
   bool _userEndedCall = false;
@@ -193,6 +194,13 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
     setState(() {});
   }
 
+  /// Toggle speaker / earpiece.
+  void _toggleSpeaker() {
+    _isSpeaker = !_isSpeaker;
+    Hardware.instance.setSpeakerphoneOn(_isSpeaker);
+    setState(() {});
+  }
+
   // ---------------------------------------------------------------------------
   // Build
   // ---------------------------------------------------------------------------
@@ -429,12 +437,10 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
             ),
             const SizedBox(width: 24),
             _CircleButton(
-              icon: Icons.volume_up,
-              label: '扬声器',
-              isActive: false,
-              onTap: () {
-                // WebRTC handles audio routing automatically
-              },
+              icon: _isSpeaker ? Icons.volume_up : Icons.hearing,
+              label: _isSpeaker ? '扬声器' : '听筒',
+              isActive: _isSpeaker,
+              onTap: _toggleSpeaker,
             ),
           ],
         );