fix(analytics): attribute referrer to the entry page only

document.referrer is fixed for the whole SPA session, so every in-app
pageview we send (#562/#565) re-credited the entry referrer — e.g. a single
visitor arriving from heise and browsing 20 anchors made heise's referrer
list show all 20 paths. Report the referrer only on the initial load (handled
by count.js); client-side route changes now send an empty referrer via a
callback. External referrers are credited to the landing page only, matching
the intuitive "where did this visit come from" meaning.

Verified with GoatCounter's get_data: with document.referrer = heise, the
landing keeps r="https://www.heise.de" while an SPA anchor view yields r="".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: raifdmueller

website/src/utils/router.js

@@ -155,7 +155,18 @@ function trackPageview() {
   }
   const gc = typeof window !== 'undefined' ? window.goatcounter : null
   if (gc && typeof gc.count === 'function') {
-    gc.count({ path: window.location.pathname, title: document.title })
+    gc.count({
+      path: window.location.pathname,
+      title: document.title,
+      // Report the referrer only on the initial load (handled by count.js).
+      // document.referrer is fixed for the whole SPA session, so without this
+      // every in-app view would be re-credited to the entry referrer (e.g.
+      // heise), inflating that referrer's page list. A callback returning ''
+      // drops the referrer for client-side navigations.
+      referrer: function () {
+        return ''
+      },
+    })
   }
 }