Implements computeImpactDeltas pure function with 8 TDD tests covering replace/add/none modes and null weight/price handling. Adds useImpactDeltas hook, categoryId to ThreadWithCandidates, and selectedSetupId state to uiStore.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
- Tests for all 4 units (g, oz, lb, kg) with known gram values
- Null and undefined handling for each unit
- Default parameter backward compatibility
- Zero and precision edge cases
- 11 pass (existing behavior), 10 fail (new unit conversion)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
