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/lib/plexDetailUi.tsx

@@ -141,7 +141,7 @@ export function DetailCustomizeMenu({
       {open && (
         <div
           ref={menuRef}
-          className="glass-menu absolute right-0 top-full z-20 mt-1 w-52 rounded-xl p-1.5 text-sm"
+          className="glass-menu absolute right-0 top-full z-menu mt-1 w-52 rounded-xl p-1.5 text-sm"
           style={{ background: "color-mix(in srgb, var(--surface) 58%, transparent)" }}
         >
           <PrefToggle label={t("plex.info.prefArtBg")} on={artBg} onClick={onToggleArtBg} />