refactor(frontend): Replace LoadingSpinner with shadcn skeleton components.

- Created AccountsSkeleton.tsx and NotificationsSkeleton.tsx components
- Updated AccountsOverview.tsx and Notifications.tsx to use skeletons
- Removed unused LoadingSpinner.tsx component
- Improved loading state UX by showing content structure

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

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

frontend/src/components/ui/progress.tsx

@@ -11,7 +11,7 @@ const Progress = React.forwardRef<
     ref={ref}
     className={cn(
       "relative h-2 w-full overflow-hidden rounded-full bg-secondary",
-      className
+      className,
     )}
     {...props}
   >
@@ -23,4 +23,4 @@ const Progress = React.forwardRef<
 ));
 Progress.displayName = ProgressPrimitive.Root.displayName;
 
-export { Progress };
+export { Progress };