Implement media library and asset picker for catalog resources

[juliuskrah]

Parent initiative: #287

Summary

Implement a media library and asset picker for catalog resources in the commerce admin.

Acceptance criteria

• Media library views browse file-backed catalog assets from live GraphQL data in the active context.
• Asset picker workflows attach or replace media references for products, ProductVariant records, collections, and categories where supported.
• Preview, selection, unavailable asset, empty library, loading, and API error states are visible.
• The picker is keyboard accessible and closes without losing unsaved form state.
• Unit/component tests cover browsing, selection, preview fallback, attachment mutation handling, and error states.

Architecture note

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