style: Update z-index in PortalDropdown component to use a constant for improved maintainability and clarity

frontend/src/components/PortalDropdown.jsx

@@ -18,6 +18,7 @@ function PortalDropdown({
   const menuRef = useRef(null);
   const [open, setOpen] = useState(false);
   const [coords, setCoords] = useState({ top: 0, left: 0, minWidth: 0 });
+  const BASE_Z = 1035; // above navbar (1030), below modal-backdrop (1040) and modal (1050)
 
   const updatePosition = () => {
     const btn = buttonRef.current;
@@ -78,7 +79,7 @@ function PortalDropdown({
       {open && body && createPortal(
         <div
           ref={menuRef}
-          style={{ position: 'absolute', top: `${coords.top}px`, left: `${coords.left}px`, zIndex: 10000, minWidth: `${coords.minWidth}px` }}
+          style={{ position: 'absolute', top: `${coords.top}px`, left: `${coords.left}px`, zIndex: BASE_Z, minWidth: `${coords.minWidth}px` }}
         >
           <div className={menuClassName} style={{ display: 'block' }}>
             {children}