Fix file preview line scrolling

[ymichael]

Summary

• find Pierre source lines inside the renderer's open shadow root
• rerun target-line scrolling when the worker-backed code view mounts or swaps to highlighted output
• center the target through the panel's vertical scroll offset without touching Pierre's horizontal code scroller
• scope target lookup and cleanup to the active file preview
• add regression coverage for delayed worker readiness, shadow-root line markup, and horizontal-scroll preservation

Testing

• pnpm exec turbo run test --filter=@bb/app --force -- --run src/components/secondary-panel/FilePreview.test.tsx (14/14)
• pnpm exec turbo run typecheck --filter=@bb/app
• pnpm exec turbo run lint --filter=@bb/app (passes with existing warnings)
• SawyerHood/dev-browser against the real dev app: clicked an assistant file://.../FilePreview.tsx#L1100 link with Pierre's inner code scroller preset to scrollLeft=240; confirmed line 1100 was selected and fully visible after vertical scrolling to scrollTop=19417, while horizontal scroll remained exactly 240

AGENT GENERATED: by GPT-5.6 Codex