fix: Apply flex-col-reverse to desktop fullscreen layout

Fixed missing flex-col-reverse in desktop sidebar mode.

Issue:
- Desktop fullscreen (sidebar mode) was missing the flex wrapper
- PageHeader appeared above SubmenuBar instead of below
- Only mobile and wp-admin layouts had the fix

Fix:
- Added flex-col-reverse wrapper to desktop fullscreen layout
- Now all three layout modes have correct header ordering:
  1. Desktop Fullscreen (Sidebar): SubmenuBar → PageHeader ✅
  2. Mobile Fullscreen: PageHeader → SubmenuBar (mobile), SubmenuBar → PageHeader (desktop) ✅
  3. Normal wp-admin: PageHeader → SubmenuBar (mobile), SubmenuBar → PageHeader (desktop) ✅

Result:
✅ Settings pages now show submenu tabs above contextual header
✅ Consistent across all layout modes
✅ Works on all screen sizes

admin-spa/src/App.tsx

@@ -474,12 +474,15 @@ function Shell() {
             <div className="flex flex-1 min-h-0">
               <Sidebar />
               <main className="flex-1 flex flex-col min-h-0 min-w-0">
+                {/* Flex wrapper: desktop = col-reverse (SubmenuBar first, PageHeader second) */}
+                <div className="flex flex-col-reverse">
                   <PageHeader fullscreen={true} />
                   {isDashboardRoute ? (
                     <DashboardSubmenuBar items={main.children} fullscreen={true} />
                   ) : (
                     <SubmenuBar items={main.children} fullscreen={true} />
                   )}
+                </div>
                 <div className="flex-1 overflow-auto p-4 min-w-0">
                   <AppRoutes />
                 </div>