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/Channels.tsx

@@ -742,7 +742,7 @@ function TagsCell({
         <div
           ref={fade.ref}
           style={fade.style}
-          className={`glass-menu absolute left-0 z-30 w-44 max-h-[264px] overflow-y-auto no-scrollbar rounded-xl p-1.5 animate-[popIn_0.16s_ease] ${
+          className={`glass-menu absolute left-0 z-chrome w-44 max-h-[264px] overflow-y-auto no-scrollbar rounded-xl p-1.5 animate-[popIn_0.16s_ease] ${
             openUp ? "bottom-full mb-1" : "top-full mt-1"
           }`}
         >