Activating a segment in read-only mode does not work

[philippotto]

Context

This impacts the selective segment visibility feature.

Expected Behavior

Even in read-only mode, the user should be able to activate a segment (e.g., so that it becomes visible when the selective segment visibility feature is enabled).

Current Behavior

Doubleclicking a segment adds that segment to the sidebar but does not activate it.

Todo

• clarify how the volume tracing layer property can be changed locally without trying to save it. ideally, we should not allow editing everything in read-only.