Add artifact registry organization sidebar menu

What does this MR do and why?

Adds the organization Artifact Registry navigation entry (monolith/S01 Step 4b) — the top-level "Artifact registry" area in the organization sidebar.

Why: now that the repositories area has its Rails mount (Step 4), it needs a discoverable navigation entry point. This adds the sidebar menu that links into the slug-scoped repositories SPA. It ships dark behind the existing artifact_registry_ui feature flag (no new flag, no changelog).

What:

• ee/lib/sidebars/organizations/menus/artifact_registry_menu.rb adds the top-level "Artifact registry" menu with a single "Repositories" item linking to the slug-scoped repositories path (built from the stub slug acme). render? gates on both the read_artifact_registry ability and the artifact_registry_ui feature flag.
• ee/lib/ee/sidebars/organizations/panel.rb inserts the menu into the EE organization panel after the Manage menu.
• Menu labels are externalized via the i18n helper under the ArtifactRegistry| namespace; locale/gitlab.pot is regenerated.

S01 ships only the activated form (the "Repositories" link); the not-activated variant (a single link to the activation page) is monolith/S10.

References

• Related issue: #604732 (closed) (Artifact Registry repositories area: Rails mount, Vue SPA shell, and navigation — monolith/S01 Steps 4, 4a, 4b)
• Builds on the merged Repositories Rails mount (Step 4, !244511 (merged)) and the shared gating seam (Step 3, !244109 (merged)).

Screenshots or screen recordings

This slice ships dark behind artifact_registry_ui. With the flag enabled, the "Artifact registry" area appears in the organization sidebar with a "Repositories" item.

Before	After

How to set up and validate locally

1. Enable the flags in a Rails console:

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

2. Visit an organization and confirm the "Artifact registry" area renders in the sidebar with a "Repositories" item linking to /o/<org>/-/artifact_registry/acme/repositories.
3. Disable artifact_registry_ui and confirm the area no longer renders.

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