feat(frontend): Transform layout to use shadcn dashboard-01 with iOS PWA safe area support.

- Replace custom layout with modern SidebarProvider/SidebarInset structure
- Add AppSidebar component using shadcn patterns with preserved account summary
- Add SiteHeader component with SidebarTrigger integration
- Install shadcn sidebar, separator, and related UI components
- Fix iOS PWA safe area issues - sidebar no longer overlaps dynamic island/notch
- Add pt-safe-top and pl-safe-left classes to sidebar and header components
- Remove legacy Sidebar.tsx and Header.tsx components
- Preserve all existing functionality: navigation, API health status, theme toggle, PWA features
- Improve mobile responsive behavior with built-in shadcn drawer patterns

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/ui/skeleton.tsx

@@ -1,4 +1,4 @@
-import { cn } from "@/lib/utils";
+import { cn } from "@/lib/utils"
 
 function Skeleton({
   className,
@@ -6,10 +6,10 @@ function Skeleton({
 }: React.HTMLAttributes<HTMLDivElement>) {
   return (
     <div
-      className={cn("animate-pulse rounded-md bg-muted", className)}
+      className={cn("animate-pulse rounded-md bg-primary/10", className)}
       {...props}
     />
-  );
+  )
 }
 
-export { Skeleton };
+export { Skeleton }