Add the changelog entry for the GNSS source window

changelog.d/1491-gnss-source-window.md

@@ -0,0 +1,3 @@
+- Let a GNSS integrity source set the layer's evidence window (`windowMs`). A
+  missing or invalid value keeps the 30-minute default, and the layer row now
+  names the window in use instead of a fixed "30 min" (samehkhamis, #1491).