fix(studio): read z-index from live iframe DOM, rename Layer to Z-index

Replace fragile regex z-index parsing with getComputedStyle on the
preview iframe elements — the same source of truth the inspector uses.

Rename "Layer" to "Z-index" in the design panel for clarity.

packages/studio/src/components/editor/PropertyPanel.tsx

@@ -323,7 +323,7 @@ export const PropertyPanel = memo(function PropertyPanel({
           </div>
           <div className="mt-3">
             <MetricField
-              label="Layer"
+              label="Z-index"
               value={String(parseInt(styles["z-index"] || "auto", 10) || 0)}
               scrub
               onCommit={(next) => onSetStyle("z-index", next)}