feat: sync git diff sidebar with scroll; sticky file headers and edge-to-edge diffs

[Dimillian]

Motivation

• Improve discoverability when scanning a multi-file diff by keeping the current file context visible and the sidebar selection in sync with the scrolled file.
• Remove the visual "card" effect so diff entries render edge-to-edge for a cleaner, denser diff view.
• Avoid scroll/selection loops when updating the sidebar as the user scrolls.

Description

• Added an onActivePathChange callback to GitDiffViewer and implemented a scroll listener that determines the top-most visible virtual item and calls onActivePathChange with the file path while avoiding update loops using activePathRef and lastScrolledPathRef (src/features/git/components/GitDiffViewer.tsx).
• Wired the selected diff path through the layout and panel by passing selectedDiffPath into GitDiffPanel from useLayoutNodes and accepting selectedPath in GitDiffPanel to add the active class to the corresponding sidebar row (src/features/layout/hooks/useLayoutNodes.tsx, src/features/git/components/GitDiffPanel.tsx).
• Removed diff card styling and made file headers sticky by updating src/styles/diff-viewer.css to render items edge-to-edge, remove rounded card borders, and add a sticky .diff-viewer-header so the filename stays visible while scrolling.
• Kept the existing virtualizer scroll-to behavior when a file is explicitly selected so clicking a sidebar file still scrolls it into view (rowVirtualizer.scrollToIndex remains in GitDiffViewer).

Testing

• Ran npm run lint and the lint step completed successfully.
• Ran npm run typecheck (tsc --noEmit) and type checking completed successfully.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

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Reviewed commit: 4d984a7448

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[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

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