[Build] feat(promo): add server validation preview system and savings badge

# Diskon Anti-PHP: Membangun Sistem Validasi Promo Super Akurat! Aku baru aja selesai --- ### Raw Commit Data ## GitHub Update **Repository:** [MKWcorp/POS-DRW-Skincare](https://github.com/MKWcorp/POS-DRW-Skincare) **Author:** MKWcorp **Date:** 5/14/2026, 7:25:15 PM **Commit:** [`0380f80`](https://github.com/MKWcorp/POS-DRW-Skincare/commit/0380f80fbdf4586343f9940922faa53d15e8d544) ### Commit Message feat(promo): add server validation preview system and savings badge - Add PromoPreviewRepository for server-side calculation - Implement preview API integration (POST /api/pos/promos/apply) - Add savings badge ('Hemat Rp X') to PromoPickerCard for eligible promos - Implement server validation in checkout flow (non-blocking) - Log PROMO_VALIDATION_MISMATCH if server vs local calculation differs >1 Rp - Activity log support for promo validation discrepancies - Preview feature: validates eligibility and calculates discount on server --- _This story was imported from historical commits._