feat(ui): 新增 PageContainer 统一页容器（P3-1）+ 作品库采用

apps/web/components/ui/PageContainer.tsx

@@ -0,0 +1,30 @@
+import type { ReactNode } from "react";
+
+import { cn } from "@/lib/ui/variants";
+
+export type PageWidth = "prose" | "default" | "wide";
+
+const widths: Record<PageWidth, string> = {
+  prose: "max-w-3xl",
+  default: "max-w-5xl",
+  wide: "max-w-6xl",
+};
+
+interface PageContainerProps {
+  children: ReactNode;
+  width?: PageWidth;
+  className?: string;
+}
+
+// 统一页容器：居中 + 一致的最大宽度与内边距节奏（替换各页手写的 mx-auto max-w-* px-6 py-10）。
+export function PageContainer({
+  children,
+  width = "default",
+  className,
+}: PageContainerProps) {
+  return (
+    <div className={cn("mx-auto px-6 py-10 sm:px-8", widths[width], className)}>
+      {children}
+    </div>
+  );
+}