feat(ui-mcp): Re-add MCP setup page (#4740)

## Description

<!-- What changed? Include screenshots if you modify the UI. -->

Clerk's `getToken` races if it is called before `isLoaded` (afaict)
(`isLoaded` is also implied by `isSignedIn`). This behavior was breaking
the MCP tool fetch logic on the new MCP setup page. This unreverts that
and ensures that the auth isLoaded is true prior to calling `getToken`

## Release note

<!-- User-facing release note. Use "None" if this does not need release
notes. -->
None.

## Migration note

<!-- Upgrade, deployment, config, schema, or compatibility notes. Use
"None" if none. -->
None.

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Co-authored-by: Sanjana Laddha <66170781+sanjanaladdha@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>