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 6c91a7f.}

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Note

Low Risk
Primarily UI-state and styling changes localized to diff rendering; risk is limited to potential regressions in scrolling or header click handling.

Overview
Adds a per-file collapse/expand toggle in DiffPanel, hiding a file’s diff body via a new diff-file-collapsed CSS rule while keeping the header visible.

Introduces diffPanelCollapse.ts helpers to key files, toggle/reset collapse state, and auto-expand a deep-linked diffFilePath; selection scrolling is deferred until after any required expansion, and header click-to-open now ignores clicks on the collapse control. Includes unit tests for the collapse helpers.

^{Written by Cursor Bugbot for commit 6c91a7f. 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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[shivamhwp]

noice