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 across message, work, and proposed-plan rows

• Introduces estimateTimelineRowHeight in timelineHeight.ts as a unified estimator replacing the previous per-message approach, covering work groups, proposed plans, working rows, and messages.
• Assistant message height now accounts for markdown structure (headings, lists, tables, fenced code blocks) via estimateAssistantMarkdownStructureBonusPx and text normalization before line-wrapping.
• Proposed plan rows get dynamic height based on collapsibility (size/line count), collapsed preview markdown, and a max collapsed preview height cap.
• Work row heights reflect entry visibility (overflow truncation vs. expansion), optional group header, and per-entry changed-file chip rows.
• Adds resolveMessagesTimelineRows in MessagesTimeline.logic.ts to produce typed MessagesTimelineRow objects; MessagesTimeline.tsx uses these for virtualizer size estimation and rendering.
• Adds a browser test suite in MessagesTimeline.browser.tsx that measures actual rendered row heights and asserts parity with the estimator across desktop/mobile viewports and expand/collapse interactions.

^{Macroscope summarized 48a41c6.}

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[juliusmarminge]

needs tests. also I think the estimation of changed files tree also need to be fixed

[huxcrux]

needs tests. also I think the estimation of changed files tree also need to be fixed

I added tests both unit and a browser render test. Let me know in case the browser test was a bit much and i can drop it.