feat(comments): improve collapse animations and address PR feedback
Animation improvements:
- Increase collapse duration to 350ms expand / 280ms collapse
- Add combined fade + size + slide transitions
- Content slides down from parent on expand, up on collapse
- Add ClipRect to prevent overflow on nested threads
- Badge now uses scale + opacity animation with easeOutBack bounce
Compact collapsed state:
- Hide comment content when collapsed (only show avatar + username)
- Move "+X hidden" badge to right side, simplified to "+X"
- Reduce padding in collapsed state
PR review fixes:
- Return unmodifiable Set from collapsedComments getter
- Add accessibility Semantics with collapse/expand hints
- Add 6 unit tests for collapse state management
Also includes dart format fixes across touched files.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>