Pick a curated image when creating a computing unit

### Task Summary

Curated images exist and administrators can manage them, but a user has no way to choose one.

An image dropdown in the create-computing-unit dialog, listing ready images and defaulting to the deployment's own.

Two things worth making clear, because both surprise people: the choice is made when the unit is created and holds for its lifetime — a running unit's image cannot be swapped — and it affects only that unit.

A user who ignores the dropdown gets exactly today's behaviour.

Done when a user can create a unit on a curated image, and one who ignores the dropdown sees no change.

Part of #8466

### Task Type

- [ ] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [ ] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other