fix: finish widget modernization audit

apps/nextjs/src/components/layout/header.tsx

@@ -18,7 +18,13 @@ interface Props {
 
 export const MainHeader = ({ logo, actions, hasNavigation = true }: Props) => {
   return (
-    <AppShellHeader maw="100vw" zIndex={201} style={{ overflowX: "hidden" }} data-app-shell-header>
+    <AppShellHeader
+      maw="100vw"
+      zIndex="var(--homarr-z-index-board-header)"
+      style={{ overflowX: "hidden" }}
+      data-advanced-focus-background
+      data-app-shell-header
+    >
       <Group h="100%" gap="xl" px="md" justify="apart" wrap="nowrap">
         <Group h="100%" align="center" style={{ flex: 1 }} wrap="nowrap">
           {hasNavigation && <ClientBurger />}