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/BackToTop.tsx

@@ -48,7 +48,7 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
   return createPortal(
     <button
       onClick={toTop}
-      className={`fixed bottom-5 right-5 z-30 inline-flex items-center justify-center w-16 h-16 rounded-full glass-card glass-hover text-fg shadow-lg hover:text-accent transition-all duration-200 ${
+      className={`fixed bottom-5 right-5 z-chrome inline-flex items-center justify-center w-16 h-16 rounded-full glass-card glass-hover text-fg shadow-lg hover:text-accent transition-all duration-200 ${
         show ? "opacity-100 translate-y-0" : "opacity-0 translate-y-3 pointer-events-none"
       }`}
     >