feat(auth): redesign Login and Setup pages with split-panel branding layout (#153)

Replace plain Card-based Login and Setup forms with a professional
split-panel layout: always-dark branding panel (dot grid texture, logo,
tagline, cyan accent line) on the left, theme-aware form on the right.
Mobile collapses to single column with compact logo header.

- Add optional admin email field on Setup for license recovery
- Backend accepts and stores admin_email in setup endpoint
- Fix data-stacks-loaded attribute forwarding (wrap in div, use string values)

frontend/src/components/EditorLayout.tsx

@@ -1009,7 +1009,8 @@ export default function EditorLayout() {
           </div>
           <h3 className="text-sm font-semibold text-muted-foreground px-4 py-2 mt-2 flex-none">STACKS</h3>
           <ScrollArea className="flex-1 px-2 pb-2">
-            <CommandList className="max-h-none overflow-visible" data-stacks-loaded={!isLoading}>
+            <div data-stacks-loaded={isLoading ? "false" : "true"}>
+            <CommandList className="max-h-none overflow-visible">
               {isLoading ? (
                 <div className="space-y-2 px-2 mt-2">
                   <Skeleton className="h-12 w-full" />
@@ -1059,6 +1060,7 @@ export default function EditorLayout() {
                 ))
               )}
             </CommandList>
+            </div>
           </ScrollArea>
         </Command>
       </div>