fix(mobile): avoid bottom-edge scroll bounce

[cachacon-ai]

Summary

• keep passive channel live updates pinned without restarting bottom-edge animations
• skip thread-tail scrolls when the newly laid-out tail is already visible
• only follow live agent activity when the reader was at its tail, using a coalesced instant correction

Validation

• cd mobile && flutter test test/features/channels/channel_detail_page_test.dart
• cd mobile && flutter analyze lib/features/channels/channel_detail_page.dart lib/features/channels/thread_detail_page.dart lib/features/channels/agent_activity/agent_activity_sheet.dart test/features/channels/channel_detail_page_test.dart

Fixes repeated iOS rubber-banding during rapid agent updates while preserving user-detached scroll positions.

[themiguelamador]

The PR fails two required mobile gates. The pinned Dart formatter rewrites agent_activity_sheet.dart, message_list.dart, and the new widget test, so dart format --output=none --set-exit-if-changed . exits nonzero. After formatting, the file-size ratchet also rejects thread_detail_page.dart at 1,004 lines against the 1,000-line cap. These failures block just mobile-check/CI even though the behavior tests pass.

I fixed both in commit f6e17b624 (branch review/pr-5911-fix): applied the repository formatter and removed the redundant four-line explanation around the self-explanatory threadTailIsVisible() guard, bringing the file to 999 lines without changing behavior.

Verified with the focused channel detail suite (122 tests), just mobile-check, all 1,400 mobile tests, and git diff --check. Per repository policy, I did not run Flutter build/run/clean/upgrade commands.