fix: redirect to /admin/clients after ending impersonation session

Closes #81

- Add window.location.href = '/admin/clients' after clearing session
  state in handleEnd so staff are sent back to the admin panel
- Add a test that verifies the redirect fires when End Session is clicked

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/portal/CustomerPortal.tsx

@@ -73,6 +73,7 @@ export function CustomerPortal() {
     }
     setSession(null);
     setSessionExtended(false);
+    window.location.href = "/admin/clients";
   }, [session]);
 
   const handleExtend = useCallback(async () => {