Implement catalog product and ProductVariant management views

[juliuskrah]

Parent initiative: #287

Summary

Implement product and ProductVariant management views for catalog operations in the commerce admin.

Acceptance criteria

• Product and ProductVariant lists support live GraphQL loading, empty states, error states, and context-aware links.
• Detail views show editable product aggregate fields and sellable ProductVariant fields with validation feedback from the API.
• Create, edit, archive or disable, and save flows use committed GraphQL mutations and clear success/error states.
• Variant relationships, identifiers, status, pricing hooks, inventory hooks, and media references are visible where available.
• Unit/component tests cover forms, validation, list/detail rendering, mutation success, and API error mapping.

Architecture note

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