Add enabled/disabled toggle for push mirrors in the UI

Problem

The push mirror UI does not expose a toggle to enable or disable an existing push mirror. This means:

• A mirror created via the API with enabled=false (or without explicitly passing enabled=true, since false is the default) cannot be enabled from the UI.
• Conversely, there is no way to temporarily disable a push mirror from the UI without deleting it.

This was discovered in !227235 (merged) (Add host_keys parameter to remote mirrors REST API), where a user reported that after creating a mirror via the API in a disabled state, there was no UI affordance to enable it.

The Update a remote mirror API already supports the enabled field, so the backend capability exists — it is only the UI that is missing this functionality.

Current UI

Missing an option to enable/disable the mirror.

Proposed UI

Proposal

Add an enable/disable toggle (e.g., a checkbox or toggle switch) to the push mirror settings UI, so users can:

1. Enable a mirror that was created in a disabled state.
2. Temporarily disable an active push mirror without deleting it.

This toggle should be available both when creating a new mirror and when editing an existing one.

Additional context

• Discovered in discussion: !227235 (comment 3206695322)
• Related MR: !227235 (merged)
• The API already supports enabled via PUT /projects/:id/remote_mirrors/:mirror_id