fix(flutter): 优化二级页面返回按钮样式

lib/core/widgets/subpage_header.dart

@@ -7,14 +7,21 @@ import '../theme/app_colors.dart';
 const double _height = 52;
 const double _heightWithSubtitle = 64;
 const double _horizontal = 14;
-const double _backSize = 36;
+const double _backSize = 40;
 const double _backGap = 12;
 const double _titleFontSize = 18;
 const double _subtitleFontSize = 12;
 const double _actionGap = 8;
-const double _iconSize = 20;
+const double _iconSize = 22;
+const double _iconWeight = 600;
+const double _borderWidth = 1;
+const double _shadowBlur = 8;
+const Color _lightShadowColor = Color(0x14000000);
+const Color _darkBackground = Color(0x59000000);
+const Color _darkShadowColor = Color(0x33000000);
+const Offset _lightShadowOffset = Offset(0, 2);
 
-/// 二级页面统一返回按钮，浅色页使用 card 圆底，深色页使用透明底。
+/// 二级页面统一返回按钮，浅色页使用白底描边，深色页使用半透明黑底。
 class AppSubpageBackButton extends StatelessWidget {
   const AppSubpageBackButton({
     super.key,
@@ -35,16 +42,40 @@ class AppSubpageBackButton extends StatelessWidget {
       height: _backSize,
       child: Tooltip(
         message: MaterialLocalizations.of(context).backButtonTooltip,
-        child: Material(
-          color: dark ? Colors.transparent : AppColors.card,
-          shape: const CircleBorder(),
-          clipBehavior: Clip.antiAlias,
-          child: InkWell(
-            onTap: onBack ?? () => Navigator.of(context).maybePop(),
-            child: Icon(
-              Icons.chevron_left_rounded,
-              color: dark ? Colors.white : AppColors.onSurface,
-              size: _iconSize,
+        child: DecoratedBox(
+          key: const Key('subpage-back-shadow'),
+          decoration: BoxDecoration(
+            shape: BoxShape.circle,
+            boxShadow: [
+              BoxShadow(
+                color: dark ? _darkShadowColor : _lightShadowColor,
+                blurRadius: _shadowBlur,
+                offset: dark ? Offset.zero : _lightShadowOffset,
+              ),
+            ],
+          ),
+          child: Material(
+            color: dark ? _darkBackground : AppColors.card,
+            shape: CircleBorder(
+              side: dark
+                  ? BorderSide.none
+                  : const BorderSide(
+                      color: AppColors.border,
+                      width: _borderWidth,
+                    ),
+            ),
+            clipBehavior: Clip.antiAlias,
+            child: InkWell(
+              customBorder: const CircleBorder(),
+              onTap: onBack ?? () => Navigator.of(context).maybePop(),
+              child: Center(
+                child: Icon(
+                  Icons.chevron_left_rounded,
+                  color: dark ? Colors.white : AppColors.onSurface,
+                  size: _iconSize,
+                  weight: _iconWeight,
+                ),
+              ),
             ),
           ),
         ),