Add artifact registry organization settings mount and navigation

What does this MR do and why?

Stands up the EE Artifact Registry organization settings area for the monolith S01 Vue app shell (Step 5):

• A slug-less settings route …/-/settings/artifact_registry and Organizations::Settings::ArtifactRegistryController#show, gated through the shared ArtifactRegistryGating concern (feature flag and read_artifact_registry access; both failures render the same 404) and declaring feature_category :artifact_registry.
• A mount helper and placeholder view rendering the #js-artifact-registry-settings element with its data-app-data payload (organization global ID + stub slug). The Vue settings shell that fills the mount is a follow-up (Step 5a).
• The organization Settings-menu "Artifact registry" item: adds the missing FOSS prepend_mod_with seam to Sidebars::Organizations::Menus::SettingsMenu and an EE override that inserts the flag/permission-gated item.

Everything is gated on the artifact_registry_ui feature flag and ships dark (default disabled), so there is no changelog entry yet.

Screenshots or screen recordings

Verified locally in a browser (flag enabled for the Default organization):

• Flag on + access → 200, page title "Artifact registry settings", the settings mount element present, and the Settings sidebar shows the "Artifact registry" item linking to the settings path.
• Flag off → the route 404s and the sidebar item is absent.

Before	After
no sidebar or route

How to set up and validate locally

1. Feature.enable(:ui_for_organizations)
2. Feature.enable(:artifact_registry_ui)
3. As an organization member, visit /o/<org>/-/settings/artifact_registry → the placeholder settings page renders; the Settings sidebar shows the "Artifact registry" item.
4. Feature.disable(:artifact_registry_ui) → the route 404s and the item is hidden.

Specs:

bundle exec rspec ee/spec/requests/organizations/settings/artifact_registry_controller_spec.rb \
  ee/spec/routing/organizations/artifact_registry_routing_spec.rb \
  ee/spec/lib/ee/sidebars/organizations/menus/settings_menu_spec.rb

MR acceptance checklist

• Tests added for the new behavior (request, routing, and EE settings-menu specs).
• Feature gated behind artifact_registry_ui (dark).
• i18n externalized; locale/gitlab.pot regenerated.
• js-routes path helpers regenerated for the new route.

Related to #604734 (closed)