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/Welcome.tsx

@@ -228,7 +228,7 @@ function Lightbox({ src, label, onClose }: { src: string; label: string; onClose
   }, [onClose]);
   return (
     <div
-      className="fixed inset-0 z-[60] grid place-items-center bg-black/80 backdrop-blur-sm p-4 sm:p-8"
+      className="fixed inset-0 z-popover grid place-items-center bg-black/80 backdrop-blur-sm p-4 sm:p-8"
       onClick={onClose}
       role="dialog"
       aria-modal="true"