refactor(client): SubScaffold 六合一(视觉零变化)+ tablet 注释订正 + 平板通知铃入口(#20)

client/lib/screens/purchase_page.dart

@@ -11,6 +11,7 @@ import '../pangolin_theme.dart';
 import '../state/payment_provider.dart';
 import '../widgets/page_body.dart';
 import '../widgets/pangolin_icons.dart';
+import '../widgets/sub_scaffold.dart';
 import '../widgets/pangolin_toast.dart';
 import '../widgets/plan_card.dart';
 import '../widgets/seg_switch.dart';
@@ -133,25 +134,11 @@ class _PurchaseScreenState extends ConsumerState<PurchaseScreen> {
       },
     ));
 
-    if (widget.embedded) return body;
-    return Scaffold(
-      backgroundColor: c.bg,
-      body: SafeArea(
-        child: Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
-          Padding(
-            padding: const EdgeInsets.fromLTRB(8, 6, 16, 10),
-            child: Row(children: [
-              IconButton(
-                onPressed: widget.onBack ?? () => Navigator.of(context).maybePop(),
-                icon: Icon(PangolinIcons.arrowLeft, size: 22, color: c.fg1),
-              ),
-              Text(t.purchaseTitle,
-                  style: PangolinText.h3.copyWith(color: c.fg1, fontWeight: FontWeight.w700)),
-            ]),
-          ),
-          Expanded(child: body),
-        ]),
-      ),
+    return SubScaffold(
+      title: t.purchaseTitle,
+      onBack: widget.onBack,
+      embedded: widget.embedded,
+      child: body,
     );
   }
 }