Break up pr-review.tsx (4,361 lines) into sub-components and hooks

[jennings]

src/browser/components/pr-review.tsx is 4,361 lines — well above the 500-line threshold. It mixes diff viewing, keyboard navigation, virtualizer logic, and data loading in one file.

Goal

Reduce pr-review.tsx to under 800 lines by extracting distinct concerns into separate files.

Planned sub-tasks

• Extract DiffViewer as a separate component
• Extract keyboard navigation and virtualizer logic into a useDiffNavigation hook

Tracking issue — do not implement directly. See sub-issues.