Add campaign list loading and error states

[joelpeace48-cell]

Show a loading spinner while fetching campaigns and an error message if the request fails. Disable or hide list until loaded.

[benedictfred]

@benedictfred has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

GM
I would love to work on this issue. I will raise a PR before the end of the day.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @benedictfred to this issue.