feat: add response diff highlight to call history

[changliuchang777]

Summary

• Add a dependency-free JSON response diff utility.
• Show response diff highlights in CallHistoryRow when a comparison response is available.
• Add focused tests for added/removed diff lines and the no-comparison case.

Fixes #279

Validation

• Compared fork branch against upstream main: ahead by 3 commits, behind by 0.
• Full local test run not performed because Git clone repeatedly stalled in this environment; changes were submitted through the GitHub Contents API.