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/SidePanel.tsx

@@ -60,7 +60,7 @@ export default function SidePanel({
       <button
         onClick={onToggleCollapse}
         aria-label={pinned ? t("sidebar.unpin") : t("sidebar.pin")}
-        className={`glass glass-hover absolute top-3 left-0 z-[34] flex flex-col items-center gap-2 px-1.5 py-3 rounded-l-none rounded-r-xl transition-opacity ${
+        className={`glass glass-hover absolute top-3 left-0 z-paneltab flex flex-col items-center gap-2 px-1.5 py-3 rounded-l-none rounded-r-xl transition-opacity ${
           expanded ? "opacity-0" : ""
         } ${count > 0 ? "ring-1 ring-accent/40" : ""}`}
       >
@@ -79,7 +79,7 @@ export default function SidePanel({
       </button>
 
       {expanded && (
-        <aside className="glass absolute top-3 bottom-3 left-0 z-[35] w-64 rounded-2xl flex flex-col overflow-hidden">
+        <aside className="glass absolute top-3 bottom-3 left-0 z-panel w-64 rounded-2xl flex flex-col overflow-hidden">
           {/* Row 1 — identity only: pin, icon, title (flexes + truncates last), count, customize.
               Module actions live on their own row below so the title always has room. */}
           <div className="flex items-center gap-1.5 px-3 py-2.5 border-b border-border/60 flex-none">