Fix chat timeline row height estimation

[huxcrux]

What Changed

Improved chat timeline virtualization height estimation by switching from coarse message-only estimates to row-aware estimates for user messages, assistant messages, work groups, proposed plan cards, and the working indicator.

Assistant row estimates now also account for markdown structure and changed-files summaries, while proposed plans and work groups use estimators closer to their actual rendered shape.

Fixes: #1035

This PR supersedes #1054.

Why

Some chat rows were being underestimated, which could cause overlapping content and visible scroll instability in virtualized threads.

UI Changes

Same as Why ^

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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