fix(diff): add line count guard to prevent large file diff from freezing UI

[ErlichLiu]

Summary

• Fixes 使用 brainstorming 时右侧文件改动面板引发应用未响应 #459 — UI freeze when opening file changes panel with large generated files
• Adds MAX_DIFF_LINES = 5000 guard in DiffView.tsx before rendering MultiFileDiff
• Files exceeding the threshold show a fallback message instead of computing the expensive Myers diff

Root Cause

@pierre/diffs MultiFileDiff internally calls createTwoFilesPatch → diffLines() (Myers O(N×D)) synchronously in useMemo, blocking the main thread. The parseDiffOptions type excludes timeout/maxEditLength, so the underlying diff library cannot be constrained.

Fix

• countLines() helper efficiently counts newlines in O(chars)
• If either old or new content exceeds 5000 lines, skip MultiFileDiff and show a clean fallback
• Matches the existing MAX_PREVIEW_CHARS = 500_000 guard pattern in DiffTabContent.tsx

Test Plan

• TypeScript compiles with no errors
• Open a large file (>5000 lines) diff — should show fallback instead of freezing
• Open a normal file diff — should render normally
• Toggle between unified/split view with large files — no freeze

🤖 Generated with Claude Code