feat(web): 前端共享基础(focusRing/proseBody/text-2xs + useBodyScrollLock + Toast撤销 + error/not-found/loading路由边界)

apps/web/app/not-found.tsx

@@ -0,0 +1,24 @@
+import Link from "next/link";
+import { FileQuestion } from "lucide-react";
+
+import { EmptyState } from "@/components/ui/EmptyState";
+import { buttonClass } from "@/lib/ui/variants";
+
+export default function NotFound() {
+  return (
+    <main className="flex min-h-screen items-center justify-center bg-bg px-6 py-16">
+      <div className="w-full max-w-md">
+        <EmptyState
+          icon={FileQuestion}
+          title="页面不存在"
+          description="你要找的内容可能已被移动或从未存在。回到作品库继续创作吧。"
+          action={
+            <Link href="/" className={buttonClass({ variant: "primary" })}>
+              返回作品库
+            </Link>
+          }
+        />
+      </div>
+    </main>
+  );
+}