fix(web): reduce chat timeline scroll twitching and overlap

[huxcrux]

What Changed

This PR reduces chat timeline scroll twitching and overlap.

Smaller and medium-sized threads now render without virtualization, which removes the late row mounting and remeasurement that were causing visible jumps while scrolling upward. Larger threads still use the standard virtualized path.

The remaining row height estimation is kept only for the cases that still need it:

grouped work/tool rows
assistant messages with changed-files summaries

Fixes #1035
Related to #988, #905, #860

Why

Older rows were being mounted and resized after entering view, which caused the timeline to twitch while scrolling and sometimes contributed to overlap.

Disabling virtualization for the thread sizes users hit most often is the simplest and most reliable fix. It removes the unstable behavior directly while still keeping virtualization for larger threads.

UI Changes

This PR improves chat scroll stability and removes visible upward-scroll twitching in affected threads.

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

Improve timeline scroll stability with accurate row height estimation

• Disables virtualization for short timelines (below MIN_ROWS_TO_ENABLE_VIRTUALIZATION) to avoid measurement overhead when it isn't needed.
• Adds estimateTimelineWorkGroupHeight in timelineHeight.ts to size work-group rows based on visible entries, chip wrapping, and expansion state.
• Adds estimateChangedFilesSummaryHeight and estimateTimelineRowHeight in MessagesTimeline.tsx to account for completion dividers and changed-files summaries in assistant message rows.
• Adds countVisibleTurnDiffTreeNodes in turnDiffTree.ts to compute visible diff-tree node counts based on directory expansion state.
• Behavioral Change: shouldAdjustScrollPositionOnItemSizeChange now triggers when remaining scroll distance is ≤ AUTO_SCROLL_BOTTOM_THRESHOLD_PX (previously only when greater), fixing auto-scroll near the bottom.

^{Macroscope summarized 38d0d3a.}

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