Feature: Replace global order limits with per-item stock limits and add qty picker popup

backend/prisma/schema.prisma

@@ -22,6 +22,7 @@ model StockItem {
   bulk1Price      Float    @default(0)
   bulk2Threshold  Float    @default(0)
   bulk2Price      Float    @default(0)
+  maxStock        Float?   // null = unlimited
 }
 
 model Order {
@@ -42,6 +43,4 @@ model Config {
   finalDepositDate String?
   cookDay          String?
   payIdInfo        String?  @default("Your PayID Here")
-  maxTotalWeight   Float?   @default(100.0)
-  maxItemWeight    Float?   @default(50.0)
 }