Update RecordingButton design with new visual style

Major design changes based on user requirements:

1. Background Style Changes:
   - Remove solid color background and shadows
   - Use semi-transparent background (alpha: 0.12) for subtle visual feedback
   - Maintain circular shape for both states

2. Icon Improvements:
   - Increase icon size to 55% of button size for better visibility
   - Use color-coded icons: blue for idle, red for recording
   - Remove white icon color, use theme-based colors instead

3. Loading Indicator Enhancement:
   - Show CircularProgressIndicator during both processing and listening states
   - Position indicator as overlay using Stack layout
   - Maintain white color for progress indicator for contrast

4. Color Logic Simplification:
   - Consolidate color logic into single iconColor variable
   - Remove complex state-based color transitions
   - Use consistent color scheme: blue (#2196F3) idle, red (#FF5252) recording
   - Darker disabled color (#212121) for better contrast

5. Layout Structure Update:
   - Replace Transform.scale animation wrapper with direct Container
   - Use Stack for layering icon and progress indicator
   - Positioned widgets for precise control over element placement

This creates a more modern, minimalist design with better visual hierarchy
and improved accessibility through higher contrast ratios.

lib/src/widgets/recording_button.dart

@@ -169,7 +169,8 @@ class _RecordingButtonState extends State<RecordingButton>
         await _speechService.stopListening();
       } else {
         await _speechService.startListening(
-            partialResults: widget.partialResults);
+          partialResults: widget.partialResults,
+        );
       }
     } catch (e) {
       widget.onError?.call(SpeechRecognitionError(
@@ -191,60 +192,60 @@ class _RecordingButtonState extends State<RecordingButton>
 
   @override
   Widget build(BuildContext context) {
-    Color buttonColor;
+    Color iconColor;
     if (!widget.enabled || !_isInitialized) {
-      buttonColor = widget.disabledColor ?? Colors.grey[400]!;
-    } else if (_isProcessing) {
-      buttonColor = _isListening
-          ? (widget.recordingColor ?? const Color(0xFFFF5252))
-              .withValues(alpha: 0.7)
-          : (widget.idleColor ?? const Color(0xFF2196F3))
-              .withValues(alpha: 0.7);
-    } else if (_isListening) {
-      buttonColor = widget.recordingColor ?? const Color(0xFFFF5252); // 红色停止按钮
+      iconColor = widget.disabledColor ?? Colors.grey[850]!;
     } else {
-      buttonColor = widget.idleColor ?? const Color(0xFF2196F3); // 蓝色录音按钮
+      iconColor = _isListening
+          ? (widget.recordingColor ?? const Color(0xFFFF5252))
+          : (widget.idleColor ?? const Color(0xFF2196F3));
     }
 
     Widget button = AnimatedBuilder(
       animation: _scaleAnimation,
       builder: (context, child) {
-        return Transform.scale(
-          scale: _scaleAnimation.value,
-          child: Container(
+        return Container(
           width: widget.size,
           height: widget.size,
           decoration: BoxDecoration(
-              color: buttonColor,
+            color: iconColor.withValues(alpha: 0.12),
             shape: BoxShape.circle,
-              boxShadow: [
-                BoxShadow(
-                  color: buttonColor.withValues(alpha: 0.3),
-                  blurRadius: _isListening ? 20 : 12,
-                  spreadRadius: _isListening ? 5 : 3,
-                ),
-              ],
           ),
           child: Material(
             color: Colors.transparent,
             child: InkWell(
               borderRadius: BorderRadius.circular(widget.size / 2),
               onTap: _toggleRecording,
-                child: _isProcessing
-                    ? SizedBox(
-                        width: widget.size * 0.4,
-                        height: widget.size * 0.4,
+              child: Stack(
+                children: [
+                  Positioned(
+                    left: 0,
+                    right: 0,
+                    bottom: 0,
+                    top: 0,
+                    child: Icon(
+                      _isListening ? Icons.stop_rounded : Icons.mic_rounded,
+                      size: widget.size * 0.55,
+                      color: iconColor,
+                    ),
+                  ),
+                  if (_isProcessing || _isListening)
+                    Positioned(
+                      left: 0,
+                      right: 0,
+                      bottom: 0,
+                      top: 0,
+                      child: SizedBox(
+                        width: widget.size,
+                        height: widget.size,
                         child: CircularProgressIndicator(
                           strokeWidth: 2,
                           valueColor:
                               AlwaysStoppedAnimation<Color>(Colors.white),
                         ),
-                      )
-                    : Icon(
-                        _isListening ? Icons.stop_rounded : Icons.mic_rounded,
-                        size: widget.size * (_isListening ? 0.35 : 0.4),
-                        color: Colors.white,
                       ),
+                    )
+                ],
               ),
             ),
           ),