Shared infrastructure and exposing permissions to the MFE

[BryanttV]

Scope

• Consume the permissions field from CourseMetadataView and make it available through a context or store.
• Create a reusable hook, e.g. useCoursePermission('courses.enroll_learners').
• Create a guard component, e.g. <PermissionGate permission="…">, that hides or disables content according to the design.
• Handle backend 403s consistently (toast or error state), in case frontend and backend disagree.
• Handle routing: if a user opens a tab directly without permission, redirect them or show a "no access" state.

Acceptance criteria

• The hook and the guard have unit tests.
• Current tab behavior doesn't change until the subtasks are implemented.

Blocked by: #486 and the design of the "no permission" states