feat: search-based code navigation with peek view

[backnotprop]

Summary

• Adds IDE-like code navigation to the review UI via ripgrep-powered symbol search
• Cmd/Ctrl+click a token in a diff to find definitions and references across the repo
• Results displayed in a VS Code-style peek panel (Dockview) below the diff: syntax-highlighted file preview on the left, grouped reference list on the right
• Backend supports language-aware definition patterns for TypeScript/JavaScript, Python, Go, and Rust with generic fallback
• Results ranked by proximity: same file, changed files, same directory, with test file demotion
• Both Bun and Pi servers implement /api/code-nav/resolve and /api/code-nav/file endpoints with full parity
• Graceful degradation when ripgrep is not installed

Closes #694

Test plan

• Open a review with /plannotator-review in a repo with local changes
• Cmd+click a token — peek panel appears below the diff with definitions and references
• Click different references in the right pane — left preview scrolls to that location
• Double-click an "in diff" result — main diff panel navigates to the file with gold line highlight
• Cmd+click a new token while peek is open — panel updates with new results
• Close peek panel via X button or Escape key — diff reclaims full height
• Test in all-files view — Cmd+click works there too
• Verify parity tests pass: bun test tests/parity/
• Verify unit tests pass: bun test packages/shared/code-nav.test.ts