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/Toaster.tsx

@@ -31,7 +31,7 @@ export default function Toaster() {
   const toasts = useSyncExternalStore(subscribe, getActiveToasts, getActiveToasts);
 
   return (
-    <div className="absolute bottom-4 left-4 z-50 flex flex-col gap-2 w-80 max-w-[calc(100vw-2rem)]">
+    <div className="absolute bottom-4 left-4 z-overlay flex flex-col gap-2 w-80 max-w-[calc(100vw-2rem)]">
       {toasts.map((toast) => {
         const { icon: Icon, color, bar } = LEVEL_STYLE[toast.level];
         return (