Improve diagnostics page with active account details

Problem

When users have multiple GitLab accounts configured in the extension, they may inadvertently have the wrong account selected as active. This can lead to features appearing unavailable when they're actually supported on the correct instance.

Example scenario:

A user has two accounts configured:

• user on gitlab.com
• root on gdk.test

If they open a gitlab.com project (which has agentic chat support) while the root account is active, agentic chat won't be available. While this behavior is technically correct (the extension is checking against the active instance), it can be confusing for users trying to troubleshoot why features aren't working.

Proposed solution

Enhance the Diagnostics page to provide clearer account information. Instead of simply stating "User is authenticated," we could display:

• Which user is currently active
• Which GitLab instance they're authenticated against

This additional context would help users quickly identify when they have the wrong account selected and understand why certain features may not be available.