chore: support variable amount of jars

Currently, Jam always displays at least five jars.
joinmarket-ng [has a config variable `mixdepth_count`](https://github.com/joinmarket-ng/joinmarket-ng/blob/618ad062cba94da318d91853c6c7cef060b059ae/jmcore/src/jmcore/config.py#L234) that can be anything between 1 and 10 (at the time of writing).

Jam should support all `mixdepth_count`s >= 1

Start by searching the TypeScript UI code for the logic that renders the fixed set of five jars, then trace how the mixdepth_count configuration is exposed to the frontend. Verify the change with mixdepth_count values from 1 through 10, including the existing default behavior, and confirm the UI renders exactly that many jars.

Written by the indexing model from the issue text.