chore: add Google tag

Author: HK-SHAO

src/app.html

@@ -5,6 +5,15 @@
 	<meta charset="utf-8" />
 	<link rel="icon" href="%sveltekit.assets%/favicon.ico" />
 	<meta name="viewport" content="width=device-width, initial-scale=1" />
+	<!-- Google tag (gtag.js) -->
+	<script async src="https://www.googletagmanager.com/gtag/js?id=G-CKFT7WE83R"></script>
+	<script>
+		window.dataLayer = window.dataLayer || [];
+		function gtag() { dataLayer.push(arguments); }
+		gtag('js', new Date());
+
+		gtag('config', 'G-CKFT7WE83R');
+	</script>
 	%sveltekit.head%
 </head>