Currency search: highlight already added or remove from the result list

User types their query, e.g. "dollar".
App displays all possible currencies matching "dollar":
- Australian dollar
- Canadian dollar
- US dollar
etc.

We need to ensure that the list contains only currencies which user can add.
We must filter already added currencies.

No files or tests are named. Start by tracing the currency search and add flow, then verify how the result list is built and how added currencies are represented. Done means an already added currency is not offered as an addable result, or its highlighting behavior is explicitly implemented.

Written by the indexing model from the issue text.