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/lib/api.ts

@@ -42,10 +42,9 @@ export const apiClient = {
     id: string,
     updates: AccountUpdate,
   ): Promise<{ id: string; display_name?: string }> => {
-    const response = await api.put<ApiResponse<{ id: string; display_name?: string }>>(
-      `/accounts/${id}`,
-      updates,
-    );
+    const response = await api.put<
+      ApiResponse<{ id: string; display_name?: string }>
+    >(`/accounts/${id}`, updates);
     return response.data.data;
   },