Add changelog entry for GA4

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chriswhong

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 Recent changes to the geojson.io application. Entries are grouped by ship date.
 
+## 2026-06-05
+
+- **Google Analytics:** Added GA4 tracking via `VITE_GA_MEASUREMENT_ID` environment variable ([#994](https://github.com/mapbox/geojson.io/pull/994))
+
 ## 2026-06-01
 
 - **Add Mobile Meta viewport Tag** - Fix for browsers properly handling mobile viewports.