analytics: fix Marlin endpoint URL double-encoding in head.html

Hugo's html/template JS-escapes string values inside <script> blocks.
Calling jsonify per-field produced quoted strings like "https://..."
which were then re-escaped, embedding literal quote characters in the
endpoint value passed to the SDK.

Fix by jsonifying the whole config dict at once and marking it safeJS
so html/template does not apply a second round of escaping.

Also extract the environment ternary out of the Marlin constructor call
for readability.

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

layouts/_partials/head.html

@@ -82,11 +82,7 @@
   {{- end -}}
   {{- if and $apiKey $endpoint -}}
     <script>
-      window.__marlinConfig = {
-        apiKey: {{ $apiKey | jsonify }},
-        endpoint: {{ $endpoint | jsonify }},
-        environment: {{ $env | jsonify }}
-      };
+      window.__marlinConfig = {{ dict "apiKey" $apiKey "endpoint" $endpoint "environment" $env | jsonify | safeJS }};
     </script>
   {{- end -}}
 {{- end -}}