feat: 为RecordingButton组件增加自定义图标和图标大小设置

- 添加 idleIcon 参数支持自定义空闲状态图标
- 添加 recordingIcon 参数支持自定义录音状态图标
- 添加 iconSize 参数支持自定义图标大小
- 保持向后兼容性，使用默认图标和大小
- 更新组件文档注释

lib/src/widgets/recording_button.dart

@@ -52,6 +52,9 @@ class RecordingButton extends StatefulWidget {
   /// 录音状态时显示的图标
   final IconData? recordingIcon;
 
+  /// 按钮图标大小
+  final double iconSize;
+
   const RecordingButton({
     super.key,
     this.speechService,
@@ -68,6 +71,7 @@ class RecordingButton extends StatefulWidget {
     this.tooltip,
     this.idleIcon,
     this.recordingIcon,
+    this.iconSize = 40.0,
   });
 
   @override
@@ -236,7 +240,7 @@ class _RecordingButtonState extends State<RecordingButton>
                       _isListening
                           ? (widget.recordingIcon ?? Icons.stop_rounded)
                           : (widget.idleIcon ?? Icons.mic_rounded),
-                      size: widget.size * 0.55,
+                      size: widget.iconSize,
                       color: iconColor,
                     ),
                   ),