fix: optimize datacenter folder structure

packages/app/src/pages/testing-only.tsx

@@ -0,0 +1,18 @@
+import { useEffect } from 'react';
+import { getDataCenter } from '@affine/datacenter';
+/**
+ * testing only when development
+ */
+
+const Page = () => {
+  useEffect(() => {
+    getDataCenter().then(dc => {
+      // @ts-expect-error global variable
+      window.dc = dc;
+    });
+  }, []);
+
+  return <div>Testing only</div>;
+};
+
+export default Page;