Fix: replace uniqueEmps reference with employees state in LeaveModal

Bug: After refactoring LeavesTab to fetch employees from /api/employees/list
(useState + useEffect), the LeaveModal was still passed employees={uniqueEmps}
but that variable no longer existed, causing JS runtime error:
"Can't find variable: uniqueEmps"

Fix: rename reference to {employees}

frontend/src/pages/holidays/Holidays.jsx

@@ -426,7 +426,7 @@ function LeavesTab() {
       {(showCreate || editing) && (
         <LeaveModal
           leave={editing}
-          employees={uniqueEmps}
+          employees={employees}
           onClose={() => { setShowCreate(false); setEditing(null) }}
           onSaved={() => { setShowCreate(false); setEditing(null); load() }}
         />