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/DownloadCenter.tsx

@@ -447,7 +447,7 @@ function QuotaUserPicker({ onPick }: { onPick: (u: { id: number; email: string }
         className={inputCls}
       />
       {open && filtered.length > 0 && (
-        <div className="absolute z-10 mt-1 w-full rounded-lg glass-menu shadow-xl overflow-hidden">
+        <div className="absolute z-base mt-1 w-full rounded-lg glass-menu shadow-xl overflow-hidden">
           {filtered.map((u) => (
             <button
               key={u.id}