Align NFS headers with existing BUI page patterns

Update migrated NFS pages to use the existing HeaderPage contract instead of extending Backstage UI, and move DevTools to real subpages with the legacy DevTools content blueprint removed.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

plugins/notifications/src/components/NotificationsPage/NotificationsPage.tsx

@@ -235,7 +235,7 @@ function NotificationsPageContent(
   if (headerVariant === 'bui') {
     return (
       <>
-        <HeaderPage title={title} subtitle={subtitle} />
+        <HeaderPage title={subtitle || title} />
         {pageContent}
       </>
     );