fix: keep file panel animation smooth by caching the highlight tree

[ping1999]

Summary

Caches the rendered syntax-highlight tree and the lightweight source-line list in FileViewer with useMemo, so unrelated re-renders no longer rebuild every token element. This removes the multi-hundred-millisecond long task that previously made the file panel's open/close animation stutter and made text selection feel laggy.

Behavior

• File content, language, theme, and line-wrapping still determine what is rendered; the cached trees are rebuilt whenever any of them change.
• Highlighted and lightweight source views render exactly the same markup as before — only redundant rebuilds are skipped.
• Selection updates during drag now skip no-op state writes: an unchanged selected line range reuses the previous state object instead of triggering a viewer-wide re-render.
• No changes to loading, error, diff, preview, or mention-line behavior.

Verification

• TypeScript: tsc --noEmit
• Lint: npm run lint -- --quiet
• Tests: 844 passed (npm test)
• Manual profiling (668-line JS file): open/close panel animation runs at full frame rate with no long tasks (previously ~500 ms long task per re-render); drag-selecting text stays at full frame rate.

Recording

• Before the fix:
  https://github.com/user-attachments/assets/6f17756c-c50f-440b-a7b3-f36966870742
• After the fix:
  https://github.com/user-attachments/assets/f9fb20e0-3b50-4631-9d42-f3bed2d84d21