feat(review): VS Code-style diff search in file tree sidebar

[backnotprop]

Summary

• Adds diff search to the code review UI — search input lives in the file tree sidebar, VS Code-style
• Search engine parses unified diff patches, finds matches across all files with line numbers and side labels (new/old/ctx)
• Results replace the file tree when a query is active, grouped by file with collapsible sections and match count badges
• Clicking a match navigates to the file and scrolls to the matching line with amber highlight (passive matches get yellow)
• Highlights penetrate @pierre/diffs shadow DOM via recursive root traversal with inline styles
• Keyboard: Cmd/Ctrl+F focuses search, Enter/F3 steps through matches, Escape clears

Based on the search engine from #344 by @sercantor — his data layer (reviewSearch.ts, reviewSearchHighlight.ts, useReviewSearch.ts) is taken verbatim. The UI integration (sidebar placement, FileTree/DiffViewer wiring) is new.

Test plan

• bun test packages/review-editor/ — all 19 tests pass
• bun run build:review && bun run build — builds clean
• Manual: open code review with multiple files, search for a term, verify results appear in sidebar
• Manual: click a result → navigates to file and scrolls to match with amber highlight
• Manual: Cmd+F focuses input, Enter steps forward, Shift+Enter steps backward, Escape clears
• Manual: clear search → file tree returns
• Manual: single-file diff → no sidebar, Cmd+F falls through to browser default

[backnotprop]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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