feat: add unit mapping functionality and confirmation dialog for unit changes in import process

backend/prisma/schema.prisma

@@ -242,3 +242,18 @@ model Nutrition {
   fiber         Float?
   product       Product @relation(fields: [productId], references: [id], onDelete: Cascade)
 }
+
+model UnitMapping {
+  id           Int      @id @default(autoincrement())
+  productId    Int
+  originalUnit String
+  preferredUnit String
+  userId       Int
+
+  product      Product @relation(fields: [productId], references: [id], onDelete: Cascade)
+  user         User    @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([productId, originalUnit, userId])
+  @@index([productId])
+  @@index([userId])
+}

backend/src/receipt-import/receipt-import.service.ts

@@ -215,7 +215,7 @@ export class ReceiptImportService {
           ],
         }
       : { isGlobal: true };
-    const [aliases, products] = await Promise.all([
+    const [aliases, products, unitMappings] = await Promise.all([
       this.prisma.receiptAlias.findMany({
         where: aliasFilter,
         orderBy: [
@@ -228,6 +228,10 @@ export class ReceiptImportService {
         where: productFilter,
         select: { id: true, name: true, canonicalName: true, categoryId: true, categoryRef: { select: { id: true, name: true } } },
       }),
+      this.prisma.unitMapping.findMany({
+        where: { userId: userId },
+        select: { productId: true, originalUnit: true, preferredUnit: true },
+      }),
     ]);
 
     return items.map((item) => {
@@ -251,11 +255,17 @@ export class ReceiptImportService {
       if (!suggestion) {
         return { ...item };
       }
+
+      // Kontrollera om det finns en enhetsmappning för produkten och användaren
+      const unitMapping = unitMappings.find((um) => um.productId === suggestion.id && um.originalUnit === item.unit);
+      const preferredUnit = unitMapping ? unitMapping.preferredUnit : item.unit;
+
       const cat = suggestion.categoryRef;
       return {
         ...item,
         suggestedProductId: suggestion.id,
         suggestedProductName: suggestion.canonicalName ?? suggestion.name,
+        unit: preferredUnit,
         ...(cat ? { categorySuggestion: { categoryId: cat.id, categoryName: cat.name, path: cat.name, confidence: 'medium' as const, usedFallback: false } } : {}),
       };
     });