[WIP] Update 'Access Denied' screen for unauthorized MCP users

[Copilot]

• Analyze review feedback about onBack function only navigating when workspaceName is truthy
• Implement option 1: Hide the back button when navigation is not possible
• Wrap Button component with conditional render {workspaceName && ...}
• Verify lint passes (pre-existing warning about unused ControlPlaneType import)
• Run code review

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}