Add response diff highlighting

[es3298]

Summary

• add a reusable JSON response diff utility for call history comparisons
• render added, removed, and changed response paths inside expanded CallHistoryRow details
• pass the next older filtered call into each row so users can compare the selected response against the previous call
• add focused tests and a short docs note for the diff behavior

Closes #279

Validation

• npx vitest run src/utils/diff.test.ts src/components/CallHistoryRow.test.tsx - 21 tests passed
• git diff --check - passed
• npm run build - currently blocked by existing syntax errors in src/App.tsx and src/pages/ApiDetailPage.tsx on upstream main