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/SyncStatus.tsx

@@ -134,7 +134,7 @@ export default function SyncStatus({
           <div
             ref={popRef}
             style={{ position: "fixed", top: pos.top, right: pos.right }}
-            className="glass-menu w-64 rounded-xl p-3 z-50 text-xs text-muted animate-[popIn_0.16s_ease]"
+            className="glass-menu w-64 rounded-xl p-3 z-overlay text-xs text-muted animate-[popIn_0.16s_ease]"
           >
             <div
               title={t("header.sync.countTooltip")}