Move visitor_id into partner_metadata (widget strips partner_donation_id)

Every.org widget forwards partner_metadata but drops partner_donation_id
from the final URL. Putting visitor_id inside the metadata JSON ensures
it reaches Every.org and appears in the CSV.

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

Author: vinogradovkonst

src/layouts/Layout.astro

@@ -332,6 +332,7 @@ import { OpenPanelComponent } from '@openpanel/astro';
     // — the widget reads params from href at init time
     const urlParams = new URLSearchParams(window.location.search);
     const metadata = {
+        visitor_id: window.__oseVisitorId,
         source_page: window.location.pathname,
         entry_page: sessionStorage.getItem('ose_entry') || window.location.pathname,
         timezone: Intl.DateTimeFormat().resolvedOptions().timeZone,