Add Manage access side panel to AR repository page

What does this MR do and why?

Adds a Manage access item to the Artifact Registry repository detail menu. It opens a non-modal side panel titled "Access for ". This is Step 1 of the repository access management plan: the entry point and the panel. Step 2 fills the panel with the Direct members card.

• The item shows only when the new artifact_registry_repository_role_management_ui flag is on for the organization and the viewer has deleteRepository. The delete verdict is a stand-in until #633009 adds an access-management verdict.
• The panel mounts in the contextual panel portal through DynamicPanel. It is a region labeled by its title. Opening it moves focus into it. Close and Esc return focus to the menu button. Esc is left to an open modal or drawer.
• The Inherited members card lists the four roles with their permission descriptions from Figma. It has no counts yet: #631412 tracks them.

The feature is dark, so there is no changelog trailer.

References

• Work item: #634667
• Plan: https://gitlab.com/gitlab-org/ops/artifact-registry/-/blob/main/docs/plans/monolith/2026-09-30-repository-access-management.md
• Spec: https://gitlab.com/gitlab-org/ops/artifact-registry/-/blob/main/docs/specs/monolith/S09-repository-access-management.md
• Design: Figma 6327:110306

Screenshots or screen recordings

Before	After

Side panel open

Icons do not render in the feature spec environment, so the menu toggle and the panel's close button appear blank.

How to set up and validate locally

1. Enable the flags for your organization in the Rails console:

   org = Organizations::Organization.default_organization
   Feature.enable(:artifact_registry_ui, org)
   Feature.enable(:artifact_registry_repository_role_management_ui, org)

2. As an Artifact Admin, open a repository detail page under the organization's Artifact Registry.
3. Open the menu next to Edit and select Manage access.
4. Check that the panel opens beside the page with the Inherited members card, and that focus is in the panel.
5. Press Esc. The panel closes and focus returns to the menu button.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #634667

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