feat(diff-panel): add per-file collapse toggle in diff panel

[huxcrux]

What Changed

Adds a per-file collapse toggle in the diff panel. Each file still starts expanded by default, but users can now collapse an individual file body from the existing diff header and expand it again inline.

The change keeps the current diff rendering flow intact, preserves click-to-open behavior for file headers, and auto-expands a targeted file when the UI deep-links to a specific diff file in the diff panel.

Fixes: #1122

Why

Large diffs in the diff panel are harder to scan when every file stays open at once. This adds a minimal control to reduce visual noise without introducing a broader diff panel redesign or changing the existing diff data flow.

UI Changes

Before:

After:

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

Add per-file collapse toggle to the diff panel

• Each file header in the diff panel now shows a chevron toggle button that collapses or expands the file's diff body, hiding content via the diff-file-collapsed CSS class.
• Collapse state is managed with a Set of file keys via helpers in diffPanelCollapse.ts; selecting a file auto-expands it if collapsed and defers scroll until after expansion.
• Header clicks still open the file in the editor unless the click target is the collapse toggle (data-diff-collapse-toggle).
• Collapse state resets when the patch changes.

^{Macroscope summarized 4948c41.}

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Note

Low Risk
Low risk UI-only change that adds per-file collapsed state and some scroll/selection side effects; potential issues are limited to diff panel interaction quirks (collapse/expand, auto-scroll) rather than data or security.

Overview
Adds a per-file collapse/expand control to the diff panel by injecting a chevron button into each FileDiff header and hiding the file body when collapsed via a new diff-file-collapsed CSS rule.

Introduces diffPanelCollapse helpers (with tests) to compute stable file keys, toggle/reset collapsed state, and auto-expand a deep-linked diffFilePath; the panel resets collapse state when the selected patch changes and avoids opening files-in-editor when the collapse button is clicked.

^{Written by Cursor Bugbot for commit 4948c41. This will update automatically on new commits. Configure here.}

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[binbandit]

Nice one!

[EverythingSuckz]

bump

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

There are 2 total unresolved issues (including 1 from previous review).

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