Add artifact registry UI feature flag

What does this MR do and why?

Establishes the shared foundation for the Artifact Registry organization UI, behind a new disabled-by-default feature flag artifact_registry_ui. The surface is unavailable until the flag is enabled and makes no Artifact Registry API or GraphQL calls.

It ships:

• artifact_registry_ui feature flag (type: wip, default_enabled: false).
• ArtifactRegistryGating controller concern: a before_action that renders 404 unless the flag is enabled and the current user can read_artifact_registry on the organization. Both the flag-off and the no-access paths return an identical not-found (enumeration-safe).
• The slug-less Activation route (Organizations::ArtifactRegistryController#index) is now gated through the concern (its previous inline read-check moved into the concern).
• An EE prepend of Organizations::OrganizationsController pushes artifact_registry_ui to the organization overview frontend (adding the missing prepend_mod seam on the CE controller).
• The organization overview EE component gates the "Go to Artifact Registry" link and empty-state copy on canReadArtifactRegistry && glFeatures.artifactRegistryUi.

References

• Feature: gitlab-org#21052
• Rollout: #605113

Related to gitlab-org#21052

Screenshots or screen recordings

UI is behind the disabled artifact_registry_ui flag. Verified live in GDK across both flag states:

State	Overview (/o/<org>/-/overview)	Activation (/o/<org>/-/artifact_registry)
flag off (default)	no "Go to Artifact Registry" link or copy	404 not-found
flag on + org access	"Go to Artifact Registry" link shown	placeholder (page title) renders, clean console

How to set up and validate locally

1. Enable the flag in the Rails console:

   Feature.enable(:ui_for_organizations)
   Feature.enable(:artifact_registry_ui)

2. As an organization member, visit the organization overview (/o/<organization_path>/-/overview) - the "Go to Artifact Registry" link appears.
3. Visit /o/<organization_path>/-/artifact_registry - the placeholder page renders.
4. Disable the flag (Feature.disable(:artifact_registry_ui)) - the link disappears and the activation route returns 404.

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