fix(flutter): 优化二级页面返回按钮样式

test/core/subpage_header_test.dart

@@ -23,7 +23,7 @@ void main() {
     expect(tester.getSize(find.byKey(const Key('subpage-header'))).height, 52);
     expect(
       tester.getSize(find.byKey(const Key('sample-back'))),
-      const Size(36, 36),
+      const Size(40, 40),
     );
     final title = tester.widget<Text>(find.byKey(const Key('subpage-title')));
     expect(title.style?.fontSize, 18);
@@ -32,6 +32,24 @@ void main() {
     final back = tester.getRect(find.byKey(const Key('sample-back')));
     final titleRect = tester.getRect(find.byKey(const Key('subpage-title')));
     expect(titleRect.left - back.right, closeTo(12, 0.5));
+    final surface = tester.widget<Material>(
+      find.descendant(
+        of: find.byKey(const Key('sample-back')),
+        matching: find.byType(Material),
+      ),
+    );
+    expect(surface.color, AppColors.card);
+    expect((surface.shape! as CircleBorder).side.color, AppColors.border);
+    final shadow =
+        tester
+                .widget<DecoratedBox>(
+                  find.byKey(const Key('subpage-back-shadow')),
+                )
+                .decoration
+            as BoxDecoration;
+    expect(shadow.boxShadow!.single.color, const Color(0x14000000));
+    expect(shadow.boxShadow!.single.blurRadius, 8);
+    expect(shadow.boxShadow!.single.offset, const Offset(0, 2));
   });
 
   testWidgets('should_use_64_height_when_subtitle_exists', (tester) async {
@@ -52,7 +70,7 @@ void main() {
     expect(find.byKey(const Key('subpage-subtitle')), findsOneWidget);
   });
 
-  testWidgets('should_use_transparent_dark_back_button', (tester) async {
+  testWidgets('should_use_translucent_dark_back_button', (tester) async {
     await tester.pumpWidget(
       MaterialApp(
         theme: AppTheme.light(),
@@ -79,8 +97,18 @@ void main() {
         matching: find.byIcon(Icons.chevron_left_rounded),
       ),
     );
-    expect(material.color, Colors.transparent);
+    expect(material.color, const Color(0x59000000));
     expect(icon.color, Colors.white);
-    expect(icon.size, 20);
+    expect(icon.size, 22);
+    expect(icon.weight, 600);
+    final shadow =
+        tester
+                .widget<DecoratedBox>(
+                  find.byKey(const Key('subpage-back-shadow')),
+                )
+                .decoration
+            as BoxDecoration;
+    expect(shadow.boxShadow!.single.color, const Color(0x33000000));
+    expect(shadow.boxShadow!.single.blurRadius, 8);
   });
 }