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/PanelGroup.tsx

@@ -39,7 +39,7 @@ export default function PanelGroup({
       ref={setNodeRef}
       style={style}
       className={`glass-card rounded-xl ${
-        isDragging ? "relative z-10 shadow-2xl ring-1 ring-accent" : ""
+        isDragging ? "relative z-base shadow-2xl ring-1 ring-accent" : ""
       } ${hidden && editing ? "opacity-50" : ""}`}
     >
       <div className="flex items-center gap-1.5 px-3 py-2">