Add settings, permissions, diagnostics, and configuration screens

[juliuskrah]

Parent initiative: #287

Summary

Add settings, permissions, diagnostics, and configuration screens to the commerce admin operator console.

Acceptance criteria

• Settings screens show active namespace, repository, store, API endpoint, feature availability, and relevant configuration states.
• Permission screens explain available and unavailable actions using authorization results from the API.
• Diagnostics show API health, GraphQL operation failures, schema/codegen status, and client environment data without exposing secrets.
• Loading, empty, unauthorized, degraded, and error states are visible across settings and diagnostics views.
• Unit/component tests cover configuration rendering, permission mapping, diagnostics status, and secret redaction.

Architecture note

gitstore-admin remains an optional add-on and consumes gitstore-api; it must not talk directly to gitstore-git-service.