- SKU is NOT NULL in the database but was nullable in wizard validation,
  causing a crash when submitted without a SKU value
- Auto-generate SKU (PRD-0001, SRV-0001, DIG-0001) when not provided
- Catch QueryException in both product forms to show friendly error
  instead of crashing on unique constraint violations
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>