Show review thread replies under their thread in the PR Summary tab

[TonybynMp4]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/web

Problem or use case

In the Summary tab, every reply to a review thread shows up as its own full comment card, next to the thread's first comment. A thread with a comment and three replies takes four cards in the list, and only the first one shows which line it's about. To follow a conversation you have to piece it back together from separate cards.

The Code tab already shows each thread as one card with its replies under it, so the same conversation looks different depending on the tab.

Proposed solution

In Summary, show each review thread as one entry: the first comment, with its replies nested below it.

• Replies move out of the top-level comment list and sit under their thread.
• Sorting and "show more" count threads instead of individual replies, so one long thread doesn't push other feedback out of view.
• Resolved threads still go into the "resolved or dismissed" group as a single entry.
• Top-level PR comments and review summaries that aren't part of a thread don't change.

Web only, no server or contract changes. Timeline already groups related comments, so it stays as is.

This builds on #15257, which adds a Resolve button to each thread's first comment in Summary. With replies grouped, each thread gets one card and one button.

Why this matters

Summary is where you read through all the feedback on a PR. When replies are separate cards, it's hard to tell which comments are new feedback and which are answers to an existing thread.

[MarconLP]

Warning

This comment was written by Claude Opus 5.5 in Claude Code, running in T3 Code. A human asked for it and read it before it was posted.

I ran into the same problem and opened #20894 for it. In the Summary tab's Comments list, each review thread becomes one item: the finding first, with its replies indented underneath and path:line shown once. A resolved thread folds as one unit, and paging never splits a reply from its finding. The Timeline gets the same grouping. The PR has before/after screenshots on a public PR.

@TonybynMp4, does this match what you had in mind? And maintainers, is this direction OK for #20894?