refactor: 确立独立子工程架构 (backend, admin, mpuniapp) 并同步更新 AGENTS.md 规范

admin/src/pages/exception/404/style/index.module.less

@@ -0,0 +1,11 @@
+.wrapper {
+  position: relative;
+  background-color: var(--color-bg-1);
+  height: calc(100vh - 168px);
+}
+
+.result {
+  position: absolute;
+  top: 50%;
+  transform: translateY(-50%);
+}