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/sheet.tsx

@@ -20,7 +20,7 @@ const SheetOverlay = React.forwardRef<
   <SheetPrimitive.Overlay
     className={cn(
       "fixed inset-0 z-50 bg-black/80  data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0",
-      className
+      className,
     )}
     {...props}
     ref={ref}
@@ -44,7 +44,7 @@ const sheetVariants = cva(
     defaultVariants: {
       side: "right",
     },
-  }
+  },
 );
 
 interface SheetContentProps
@@ -79,7 +79,7 @@ const SheetHeader = ({
   <div
     className={cn(
       "flex flex-col space-y-2 text-center sm:text-left",
-      className
+      className,
     )}
     {...props}
   />
@@ -93,7 +93,7 @@ const SheetFooter = ({
   <div
     className={cn(
       "flex flex-col-reverse sm:flex-row sm:justify-end sm:space-x-2",
-      className
+      className,
     )}
     {...props}
   />
@@ -135,4 +135,4 @@ export {
   SheetFooter,
   SheetTitle,
   SheetDescription,
-};
+};