Display reply tree with indentation and branch lines on the post detail screen

[joelmuraguri]

Is your feature request related to a problem? Please describe.
PostDetailScreen renders a flat LazyVerticalStaggeredGrid of TimelineItem entries. The items block inside it handles TimelineItem.Thread, TimelineItem.Loading, and TimelineItem.Empty but has no handling for TimelineItem.ReplyTree produced by the domain layer. Beyond the missing handler, the screen has no visual concept of indentation to communicate reply depth, no sibling branch lines to show which posts share a parent, and no progressive disclosure mechanism for deep or wide reply branches.

Describe the solution you'd like
A new composable ReplyTreeItem that accepts a List<ReplyNode> and renders the full branching tree. It replaces the role that ThreadedPost plays for TimelineItem.Thread but handles branching rather than a linear chain.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.