Use Pierre renderer and virtualization for diffs

[itsjling]

Summary

• Replace the custom diff row renderer with @pierre/diffs.
• Add virtualized scrolling for large diffs.
• Preserve valid shortened patches by trimming complete hunks and recalculating hunk counts.
• Add coverage for shortened patch parsing.

Testing

• Not run (not requested)

[itsjling]

@codex review

[chatgpt-codex-connector]

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