feat: adopt material 3 theme and billing adjustments

lib/widgets/common/layout/page_container.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/material.dart';
+import '../../../theme/ui_constants.dart';
+
+/// 페이지 공통 좌우 패딩과 최대 폭을 보장하는 래퍼
+class PageContainer extends StatelessWidget {
+  final Widget child;
+  final EdgeInsetsGeometry? padding;
+  final double maxWidth;
+
+  const PageContainer({
+    super.key,
+    required this.child,
+    this.padding,
+    this.maxWidth = 720,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Align(
+      alignment: Alignment.topCenter,
+      child: ConstrainedBox(
+        constraints: BoxConstraints(maxWidth: maxWidth),
+        child: Padding(
+          padding: padding ??
+              const EdgeInsets.symmetric(
+                horizontal: UIConstants.pageHorizontalPadding,
+              ),
+          child: child,
+        ),
+      ),
+    );
+  }
+}