docs: fix Regular Store payment — wallet-only, no IPG/ZarinPal

- Regular Store: Balance deduction only (SubmitShopBuyOrderCommandHandler)
  NO ZarinPal redirect, NO payment gateway
- Discount Store: DiscountBalance + ZarinPal IPG for remainder
  Added gatewayAmount=0 edge case (discount covers 100%)
- VAT: Both stores use 9% (ShopVAT=0.1 is stale/unused constant)
- Added ZarinPal usage scope note (only: Discount Store, Wallet Top-up, Package Purchase)
- Fixed BUSINESS-02: sections 2, 5.2, 5.3, 7
- Fixed BUSINESS-03: store comparison chart
- Fixed OVERVIEW-01: user journey + both store flowcharts
- Fixed OVERVIEW-03: timeline entry

business/BUSINESS-03-ECOMMERCE-STORES.md

@@ -11,9 +11,9 @@
 flowchart LR
     subgraph RS["Regular Store — /store"]
         R1["همه کاربران"]
-        R2["پرداخت 100% نقدی"]
+        R2["پرداخت از Balance کیف‌پول"]
         R3["قیمت عادی"]
-        R4["VAT = 10%"]
+        R4["VAT = 9%"]
     end
 
     subgraph DS["Discount Store — /discount-store"]