fix(frontend): open the variable expiration handler template in a new tab

The "Create a handler" button hands its script template to /scripts/add through
the URL hash, which `load` reads from `window.location`. A client-side navigation
leaves the previous page's hash in place, so the payload was dropped and the
editor opened on the default template at a `u/` path. `target="_blank"` forces a
full page load, matching the success handler's "Create from template" button.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/routes/(root)/(logged)/workspace_settings/+page.svelte

@@ -2153,6 +2153,7 @@ export async function main(
 }
 `
 												})}
+											target="_blank"
 										>
 											Create a handler
 										</Button>