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/PlexBrowse.tsx

@@ -231,7 +231,7 @@ export default function PlexBrowse({
     // The fallback is portaled for the same reason the player itself is: it is a fixed
     // full-viewport overlay, and this tree renders inside the page scroller.
     return (
-      <Suspense fallback={createPortal(<div className="fixed inset-0 z-50 bg-black" />, document.body)}>
+      <Suspense fallback={createPortal(<div className="fixed inset-0 z-overlay bg-black" />, document.body)}>
         <PlexPlayer itemId={sub.view.id} queue={sub.view.queue} onClose={sub.back} />
       </Suspense>
     );
@@ -924,7 +924,7 @@ function PlexShowView({
         <>
           {/* Hero */}
           <div className="glass relative rounded-2xl p-4 sm:p-5 mb-8 flex gap-4 sm:gap-5">
-            <div className="absolute right-3 top-3 z-10">
+            <div className="absolute right-3 top-3 z-base">
               <DetailCustomizeMenu
                 artBg={artBg}
                 onToggleArtBg={toggleArtBg}
@@ -1121,7 +1121,7 @@ function PlexSeasonView({
       ) : (
         <>
           <div className="glass relative rounded-2xl p-4 sm:p-5 mb-8 flex gap-4 sm:gap-5">
-            <div className="absolute right-3 top-3 z-10">
+            <div className="absolute right-3 top-3 z-base">
               <DetailCustomizeMenu
                 artBg={artBg}
                 onToggleArtBg={toggleArtBg}