fix(dev-portal): use tiered z-index instead of isolation for header stacking

Replace the blanket isolation: isolate on Styled.Main with a moderate
explicit z-index (10) on Styled.Head. Isolation trapped ALL descendant
stacking inside Main, including deliberate fullscreen overlays/scrims apps
render when opening sidepanels, preventing them from ever covering the
portal header even when that's the intended UX.

The tiered z-index still beats small incidental leaks (like the original
app-admin z-index: 2 regression) while staying below the z-index range
conventionally used by real overlay/backdrop components, so intentional
fullscreen app overlays can still render above the header.

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

Author: chdefrene

.changeset/portal-header-stacking-fix.md

@@ -2,4 +2,4 @@
 "@equinor/fusion-framework-dev-portal": patch
 ---
 
-Fixes the portal chrome (Header/TopBar and its ContextSelector dropdown) being hidden behind a loaded app's content when the app sets its own internal `z-index`. The dev-portal's header region now establishes an explicit stacking context above the main content area, and the main content area isolates any stacking context an app creates internally so it can no longer escape and cover the portal chrome.
+Fixes the portal chrome (Header/TopBar and its ContextSelector dropdown) being hidden behind a loaded app's content when the app sets its own internal `z-index`. The dev-portal's header region now uses an explicit `z-index` high enough to beat small, incidental z-index values apps tend to use for ordinary layout elements, while staying below the conventional z-index range used by real overlay/backdrop components — so an app's intentional fullscreen scrim (e.g. opening a sidepanel) can still render above the header when desired.

packages/dev-portal/src/Router.tsx

@@ -19,26 +19,25 @@ const Styled = {
   Head: styled.section`
         grid-area: head;
         /**
-         * Establish an explicit stacking context above the default (auto)
-         * layer so the portal chrome (Header/TopBar and its ContextSelector
-         * dropdown) always paints above the loaded app's content, regardless
-         * of any z-index the app sets internally on its own elements.
+         * Give the portal chrome (Header/TopBar and its ContextSelector
+         * dropdown) an explicit stacking context comfortably above the
+         * small, incidental z-index values apps tend to use for ordinary
+         * layout elements (sticky headers, dropdowns, etc.), so those can
+         * never leak above the portal chrome. Deliberately kept well below
+         * the z-index range conventionally used by real overlay/backdrop
+         * components (e.g. modal libraries typically use 1000+), so an
+         * app's intentional fullscreen scrim (e.g. opening a sidepanel)
+         * can still render above the header when that's the desired UX.
          */
         position: relative;
-        z-index: 1;
+        z-index: 10;
     `,
   Main: styled.section`
         grid-area: main;
         overflow: auto;
         position: relative;
         max-width: 100%;
         display: grid;
-        /**
-         * Isolate the loaded app's stacking context so any z-index it sets
-         * internally is contained within this region and can never escape
-         * to compete with the portal chrome rendered in the Head section.
-         */
-        isolation: isolate;
     `,
 };