Implement collection and category management views

[juliuskrah]

Parent initiative: #287

Summary

Implement collection and category management views for catalog operations in the commerce admin.

Acceptance criteria

• Collections and category taxonomies can be browsed, searched, and opened from context-aware admin routes.
• Category hierarchy, collection membership, and product assignment states render from live GraphQL data.
• Create and edit forms enforce API validation and show loading, empty, conflict, and error states.
• Changes preserve hierarchy constraints and refresh affected catalog operations without manual reloads.
• Unit/component tests cover hierarchy rendering, form validation, empty states, GraphQL errors, and successful updates.

Architecture note

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