Create gitstore-admin compatibility test suite

[juliuskrah]

Parent initiative: #287

Summary

Create a compatibility test suite that keeps the commerce admin aligned with GitStore API behavior and expected operator console workflows.

Acceptance criteria

• The admin package has CI coverage for npm run build, npm run test, npm run codegen, and npm run test:e2e.
• Unit and component tests cover auth, route guards, table controls, forms, filters, saved views, and error mapping.
• Playwright coverage exercises login, namespace/store switching, catalog browsing, product and ProductVariant detail, collections and categories, media selection, order and customer views where backend blockers are available, and diagnostics.
• GraphQL operation/codegen checks fail when committed queries or mutations drift from the GitStore schema.
• Test fixtures avoid direct service access and exercise the admin through gitstore-api boundaries.

Architecture note

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