fix(frontend): stop group modals clearing selection on background refetch

The reset effect in GroupAddClientsModal and GroupRemoveClientsModal
depended on the memoized rows, which are rebuilt whenever GroupsPage
re-renders because candidates/members are inline-filtered arrays. The
5s client-list poll re-renders the page, so any selection made in the
modal was wiped a few seconds later. Reset only when the modal opens.

frontend/src/pages/groups/GroupAddClientsModal.tsx

@@ -50,7 +50,7 @@ export default function GroupAddClientsModal({
     if (!open) return;
     setSelectedEmails([]);
     setSearch('');
-  }, [open, rows]);
+  }, [open]);
 
   const filteredRows = useMemo(() => {
     const q = search.trim().toLowerCase();

frontend/src/pages/groups/GroupRemoveClientsModal.tsx

@@ -48,7 +48,7 @@ export default function GroupRemoveClientsModal({
     if (!open) return;
     setSelectedEmails([]);
     setSearch('');
-  }, [open, rows]);
+  }, [open]);
 
   const filteredRows = useMemo(() => {
     const q = search.trim().toLowerCase();