Add root loading component

src/app/loading.tsx

@@ -0,0 +1 @@
+export { default } from '@/components/PageSpinner';

src/components/PageSpinner.tsx

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+import clsx from 'clsx/lite';
+import Spinner from './Spinner';
+import SiteGrid from './SiteGrid';
+
+export default function PageSpinner() {
+  return (
+    <SiteGrid contentMain={
+      <div className={clsx(
+        'flex justify-center items-center',
+        'w-full min-h-[20rem] sm:min-h-[30rem]',
+      )}>
+        <Spinner
+          size={24}
+          color="light-gray"
+        />
+      </div>
+    } />
+  );
+}