Add loading skeleton to agent list page

[wisdomqin]

Description

When the agent list page loads, there is a brief blank screen before data appears. Adding a skeleton loading state would improve the user experience.

What to do

1. Create a skeleton component in frontend/src/components/
2. Show the skeleton in the agent list page while data is loading
3. Match the existing dark theme style

Resources

• Agent list page: frontend/src/pages/
• Styles: frontend/src/index.css
• Data fetching uses TanStack React Query (useQuery with isLoading state)