fix(admin-web): keep app header above content stacking contexts

Promote the header to z-index 100 and create an isolated stacking
context on the main layout so sticky/positioned content (drawers,
overlays) no longer sits above the topbar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/admin-web/src/layouts/AppShell.vue

@@ -353,6 +353,7 @@ async function handleLogout(): Promise<void> {
 
 .admin-main-layout {
   margin-left: 250px;
+  isolation: isolate;
 }
 
 .admin-main-layout--blocked {
@@ -387,7 +388,7 @@ async function handleLogout(): Promise<void> {
   display: flex;
   align-items: center;
   justify-content: space-between;
-  z-index: 1;
+  z-index: 100;
   box-shadow: 0 1px 4px rgba(0, 21, 41, 0.08);
 }
 
@@ -552,6 +553,8 @@ async function handleLogout(): Promise<void> {
 }
 
 .admin-content {
+  position: relative;
+  z-index: 0;
   margin: 24px;
   min-height: auto;
 }

apps/admin-web/src/styles.css

@@ -140,7 +140,6 @@ html, body, #app {
 
 .ant-layout-header {
   box-shadow: 0 1px 4px rgba(0,21,41,0.08); /* slight shadow for topbar */
-  z-index: 1;
 }
 
 .ant-card {