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/AddToPlaylist.tsx

@@ -130,7 +130,7 @@ export default function AddToPlaylist({
           <div
             ref={panelRef}
             style={{ position: "fixed", top: coords.top, left: coords.left, width: 240 }}
-            className="z-50 glass-menu rounded-xl p-2 shadow-2xl"
+            className="z-overlay glass-menu rounded-xl p-2 shadow-2xl"
             onClick={(e) => e.stopPropagation()}
           >
             <div className="text-xs text-muted px-1.5 pb-1.5">