Improved threaded stacktrace UI

## Problem

The current UI for threaded stack traces is a bit disjointed and the UX when using it is sub-par. Some feedback we've received:

- Main thread is not identified in the selector: https://github.com/getsentry/team-mobile/issues/198
- Scrolling through threads to find what you're looking for is quite difficult: https://github.com/getsentry/sentry/issues/20746
- It is difficult to know from the selector which threads actually have interesting stack traces (many threads have empty stack traces or stack traces with no in-app frames)

Internal: [see here for an example issue with threaded stack trace](https://demo.sentry.io/issues/4590863338/)



## Solution

TBD