feat(web): scroll to exact line when focusing a key change

[dastratakos]

Summary

When a user focuses a key change, the view now scrolls to the exact diff line instead of just scrolling to the file header. The hunk highlight overlay also measures line rects more accurately across number and content columns.

Changes

• Add scrollToLine and cancelScrollToLine to FileDiffListHandle, with logic to auto-expand collapsed files, wait for shadow DOM rendering, and scroll the target line into view
• Extract getHighlightLineRect and helpers (findLineRow, findLineNumberElement) in hunk-highlight-overlay.tsx for accurate rect measurement across line number + content columns
• Update ChapterDetailContent to call scrollToLine instead of scrollToFile when focusing a key change
• Add unit tests for getHighlightLineRect and the helper functions

Testing

• Typecheck, lint, and all 160 tests pass
• Manual verification needed for scroll-to-line behavior in the browser

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[gemini-code-assist]

Code Review

This pull request enhances the diff viewer by implementing a robust scrollToLine functionality that handles collapsed files and asynchronous shadow DOM rendering. It also introduces utility functions for more accurate line highlighting by including the line number gutter in the bounding rectangle. Feedback focuses on optimizing DOM element lookups in large diffs and improving the lifecycle management of scroll-related observers and timers to ensure immediate execution and proper cleanup.

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