Handling empty and duplicate IDs

- Strictly speaking, it is good to make users aware of duplicate or empty IDs.
- IDs are only required for linking at this stage.
- The Executor currently assumes unique and existing IDs to ensure everything can be linked.
- The Preview should work in all possible cases, and a missing ID is probably avoidable easily.
- Not sure for duplicates, though.

## Plan

- [x] For now, generate IDs in JS so empty IDs will not cause the preview to not work. #205
- [ ] Discuss later how this should be synchronized with python or be documented somewhere (JS code can also be exported).

Start by reviewing the Preview and Executor behavior described in the issue, along with the JavaScript ID generation added in #205. Clarify how duplicate IDs should be handled and whether synchronization with Python or documentation is required; done means the behavior and its synchronization or documentation path are agreed and addressed.

Written by the indexing model from the issue text.