Fix chat bubble blink and tail-follow scroll

[YeungKC]

Summary

• Keep message blink painting on actual bubble, media, quote, action card, and action button surfaces.
• Preserve bottom tail-follow animation for newly appended messages without emitting a visible jump first.
• Add focused widget coverage for bubble blink surfaces and tail-follow scroll animation.
• Add opt-in chat scroll trace logging behind MIXIN_CHAT_SCROLL_TRACE for future scroll diagnosis.

Tests

• flutter test test/widgets/message/message_context_test.dart
• flutter test test/ui/home/chat/chat_scroll_coordinator_test.dart
• flutter analyze affected chat bubble and scroll files