Interests and Topic feature modules should be merged

Previously we had an authors module so the module structure was:

- interests
- authors
- topics

Now that we no longer have authors, the nested structure adds complexity for no benefit, making it especially difficult to understand the navigation graph. Suggest merging `:topics` into `:interests` (whilst keeping the `Topic` model).

This would enable `InterestsListDetailScreen` which is currently in the `:app` module to be moved into the feature module.

Start by tracing the :interests, :topics, and :app modules, including InterestsListDetailScreen and the Topic model, to understand the current navigation graph and module dependencies. Done means merging :topics into :interests while keeping Topic and moving InterestsListDetailScreen out of :app without breaking the feature structure.

Written by the indexing model from the issue text.