Add artifact registry repositories SPA shell

What does this MR do and why?

Adds the organization Artifact Registry repositories-area Vue SPA shell (monolith/S01 Step 4a) — the Platform foundation later AR slices render inside.

Why: the repositories area needs a Vue single-page app shell before any slice can render inside it: the SPA bootstrap, the router scaffolding, and the page-entrypoint wiring that mounts it. It registers no slice routes and makes no AR call, and stays dark behind the existing artifact_registry_ui feature flag (no new flag, no changelog).

What:

• pages/repositories/index.js reads the mount dataset, builds the history-mode router from the controller-supplied slug-scoped base path, attaches the shared createDefaultClient Apollo provider, and mounts the shell.
• pages/repositories/app.vue hosts the router outlet in a focusable container and moves focus to it on client-side route changes.
• router/index.js registers the catch-all in-SPA not-found route and the afterEach document-title hook; it registers no slice routes.
• components/not_found.vue renders the in-SPA not-found empty state (heading level 1); utils.js updateDocumentTitle keeps the document title in sync with the active route.
• pages/organizations/artifact_registry_repositories/index/index.js is the per-page auto-loaded entrypoint that calls initArtifactRegistryRepositories so the shell mounts in the running app.
• Shell strings are externalized via the i18n helper. Jest/VTU specs cover the router, the shell, and route-change focus; an RSpec feature spec drives the rendered view in a browser to guard the page-entrypoint wiring.

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 and registers no slice routes, so the mounted shell renders the in-SPA "Page not found" empty state until later slices add routes.

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 /o/<org>/-/artifact_registry/acme/repositories — the Vue shell mounts (focusable container).
3. Visit /o/<org>/-/artifact_registry/acme/repositories/does-not-exist — the Rails *vueroute catch-all serves the SPA and the in-SPA "Page not found" fallback renders.
4. Visit /o/<org>/-/artifact_registry/notacme/repositories — a slug other than acme returns a Rails 404.

MR acceptance checklist

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