Fix IAM actions dropdown rendering behind System Settings card

Add relative z-10 to the Users Card so its stacking context sits above
the sibling System Settings Card. Without this, the absolutely-positioned
dropdown menu was painted behind the later sibling in DOM order.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/admin/IAMPage.tsx

@@ -123,8 +123,8 @@ export default function IAMPage() {
         />
       </div>
 
-      {/* User table */}
-      <Card>
+      {/* User table — relative z-10 so action dropdowns render above sibling cards */}
+      <Card className="relative z-10">
         <CardHeader className="flex-row items-center justify-between flex-wrap gap-2 md:gap-3">
           <div className="flex items-center gap-2">
             <div className="p-1.5 rounded-md bg-accent/10">