fix(dev-portal): align stacking with app portal

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: chdefrene

.changeset/dev-portal_fix-stacking.md

@@ -0,0 +1,5 @@
+---
+"@equinor/fusion-framework-dev-portal": patch
+---
+
+Match `app-portal` stacking behaviour.

packages/dev-portal/src/Router.tsx

@@ -18,11 +18,14 @@ const Styled = {
     `,
   Head: styled.section`
         grid-area: head;
+        z-index: 2;
     `,
   Main: styled.section`
         grid-area: main;
+        --header-height: 48px;
         overflow: auto;
         position: relative;
+        z-index: 1;
         max-width: 100%;
         display: grid;
     `,