desktop: ui / ux for model managing

We should think about how models are managed in transcribee desktop:
* is there an explicit flow for downloading / deleting models?
* how are different models "bundled" together (sad, whisper, parakeet, diarization)?

Overall, I think it is fine to manage models implicitly for the time being until we have figured out a bit more explicitly how the redone transcription works.

However, there should at least be some indication that a model is currently being downloaded.

Start by reviewing the desktop transcription flow and how model downloads are currently triggered. Clarify the model bundling and explicit download/deletion questions before choosing a direction. Done should include at least a visible indication when a model is being downloaded, with the broader management behavior documented or agreed.

Written by the indexing model from the issue text.