refactor(z-index): name the stacking layers as a token scale

Replace ~48 magic z-index values (z-10..z-[9999] + 2 inline zIndex) with named
tailwind zIndex tokens (base/menu/chrome/paneltab/panel/rail/overlay/popover/
tooltip/tuner). Values are UNCHANGED — a rename, not a renumber — so layering is
identical; the names give one source of truth for the stack.

frontend/src/components/Modal.tsx

@@ -52,7 +52,7 @@ export default function Modal({
 
   return createPortal(
     <div
-      className="fixed inset-0 z-50 flex items-center justify-center p-4 sm:p-6 bg-black/70 backdrop-blur-sm"
+      className="fixed inset-0 z-overlay flex items-center justify-center p-4 sm:p-6 bg-black/70 backdrop-blur-sm"
       onMouseDown={(e) => {
         pressedOnBackdrop.current = e.target === e.currentTarget;
       }}