Add line-wrap toggle to the file diff panel

[pufit]

Adds a line-wrapping toggle to the side panel's file diff inspector.

What

• New WrapText icon button in the diff detail header — toggles long lines between horizontal scrolling (default, unchanged) and soft wrapping.
• Preference persists across sessions via localStorage (nerve_diff_wrap).
• Plumbing: useDiffOptions() accepts an optional { wrap } flag mapped to @pierre/diffs' overflow: 'wrap' | 'scroll' option; DiffView forwards a wrap prop. Other diff renders (e.g. Edit tool blocks) are untouched — the no-arg call keeps the library default (scroll).

Testing

• npm run build clean, full pytest suite passes (1349 passed).
• Verified in the browser: data-overflow flips scroll ↔ wrap in the diffs shadow root, long lines visibly wrap, preference survives reopening files, active state styled with text-accent.

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