Android: scroll-down (jump to latest) arrow missing inside threads

## Summary
On Android, the scroll-down ("jump to latest") arrow does not appear inside thread views. It works in the main channel view, but once inside a thread there is no affordance to jump back to the newest message.

## Steps to reproduce
1. Open Buzz Mobile on Android (0.8.0).
2. Open a channel and enter a thread with enough messages to scroll.
3. Scroll up within the thread.

**Expected:** a scroll-down arrow appears to jump to the latest message (as in channel view).
**Actual:** no arrow is shown inside threads.

## Source
Reported by @thesosana on X during the v0.8.0 launch feedback sweep: https://x.com/thesosana/status/2087329943185191414 (also asked the team to review open Android PRs).

Triage thread: buzz-mobile channel, X feedback sweep 2026-08-09→12.

## Related
- #4427 (Android: opening a channel does not scroll to newest message)
- #5369 (thread scroll controller starts at first message)