[Feature Request]: Design UI of the merge_ui

### Feature description

Linked with: https://github.com/insightsengineering/teal.transform/issues/17

1. Provide wireframes and description of how relationship between datasets should be displayed in the `merge_ui`.
Inspiration:
- `dm` package https://krlmlr.github.io/dm/
- random image showing foreign keys in a database

2. If `merge_srv` fails, where errors should be displayed

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