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

Note

Fix chat timeline row height estimation using content-aware logic

• Replaces per-kind height estimators with a single estimateTimelineRowHeight in timelineHeight.ts, covering work, working, proposed-plan, and message rows.
• Assistant message height now includes bonuses for markdown structure (headings, lists, tables, code blocks) and diff summary tree size.
• Work row height varies with entry count, expansion state, and changed-file chips; proposed-plan rows reflect collapsed preview vs. full height.
• MessagesTimeline attaches assistantDiffSummary directly to each message row object, removing the per-render map lookup.
• Behavioral Change: virtualizer row height estimates will change for most row types, which may cause minor scroll position shifts in existing chat sessions.

^{Macroscope summarized bed6475.}

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