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/expand toggle to the diff panel

• Adds a collapse/expand toggle button in each file header in DiffPanel.tsx, tracking collapsed state as a Set of file keys.
• Collapsed files hide their code content via a new diff-file-collapsed CSS rule (display: none on <pre> and virtualizer buffer).
• Collapse state resets when a new patch is selected; selecting a collapsed file auto-expands it and skips scroll-into-view when collapsed.
• Extracts shared helpers (toggleCollapsedDiffFile, resetCollapsedDiffFiles, expandCollapsedDiffFileForPath, buildFileDiffRenderKey, resolveFileDiffPath) into diffPanelCollapse.ts with full Vitest coverage.
• Behavioral Change: clicking the file header now opens the editor unless the click target is the new collapse toggle button.

^{Macroscope summarized 523a0b1.}

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[binbandit]

Nice one!