[Enhancement]: Improve UI/UX when API requests are loading

### Is your feature request related to a problem?

There is a placeholder text when API requests are made. For example, on the homepage it says "Loading catalog..." which is not aesthetically pleasing: https://simboard-dev.e3sm.org/

### Describe the solution you'd like

Use async loading spinners, icons, etc. -- fill this in more detail later.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start at the homepage shown in the issue's linked SimBoard URL and identify where the “Loading catalog...” placeholder is rendered during API requests. Define the spinner or icon behavior and verify that loading states no longer use the placeholder; the issue does not name files or tests.

Written by the indexing model from the issue text.