Fix widget modal positioning to avoid header overlap

Add data-modal-y-offset and data-modal-z-index parameters to the Kapa widget script to ensure the modal appears below the header and on top of other page elements.

Author: ebembi-crdb

src/current/_includes/head.html

@@ -151,6 +151,8 @@
   data-kapa-branding-hide="true"
   data-modal-title="CockroachDB AI"
   data-modal-disclaimer="This AI bot is experimental. Don't rely on it for production workloads. Always consult the official CockroachDB documentation. Interactions with this AI bot may be monitored or recorded, and your use of the AI bot is subject to the Cockroach Labs' [Terms of Use](https://www.cockroachlabs.com/website-terms-of-use/) and [Privacy policy](https://www.cockroachlabs.com/privacy/)."
+  data-modal-y-offset="18vh"
+  data-modal-z-index="1000"
 ></script>
 
 <style>