add Google Analytics 4 (gtag.js) tracking

Measurement ID G-2WWKKGDL6H wired via site.analytics.measurementId.
Layout conditionally injects the gtag.js loader + dataLayer init +
config call when analytics.measurementId is set, mirroring the
AdSense conditional pattern.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/site.js

@@ -7,4 +7,7 @@ export const site = {
     enabled: true,
     publisherId: '9763775565109295',
   },
+  analytics: {
+    measurementId: 'G-2WWKKGDL6H',
+  },
 };