[TASK] Implement loading screen for Workspaces

[thaorell]

Certification

• I certify I am an Epic Owner for Kubeflow Notebooks 2.0 and expected to create planning-related issues.

Description

Currently, a race condition can occur where the UI attempts to make an API call before the namespace data is available, leading to an empty state and a 404 error.
When the component mounts and the namespace data is not yet available, a loading spinner or skeleton component should be rendered. The workspace list should only attempt to render and make its specific API calls once the namespace data has been successfully loaded into the component's state.

Acceptance Criteria

• The Workspaces.tsx component displays a clear loading indicator while namespaces are being loaded.
• The workspace list only renders after the namespace data is available.
• The 404 error related to the premature API call is no longer reproducible.

[andyatmiami]

/close

resolved in #604

[google-oss-prow]

@andyatmiami: Closing this issue.

In response to this:

/close

resolved in #604

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