Add inventory and pricing operation surfaces for variants

[juliuskrah]

Parent initiative: #287

Summary

Add inventory and pricing operation surfaces for ProductVariant records in the commerce admin.

Acceptance criteria

• Variant detail and list views expose inventory and pricing summaries from live GraphQL data.
• Business users can edit supported inventory and pricing fields with API validation, confirmation, and clear error handling.
• Batch-friendly controls are designed for repeated catalog operations without hiding per-variant failures.
• Loading, empty, stale, unauthorized, and conflict states are rendered in the operator console.
• Unit/component tests cover numeric validation, mutation success, partial failure mapping, reload behavior, and permission states.

Architecture note

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