Refactor SaleSection component to utilize direct property assignment for betterPrices, enhancing reactivity in Vue3. Update inventory mapping to ensure betterPrices is always an array.

frontend/src/components/falukant/SaleSection.vue

@@ -290,6 +290,8 @@
           this.inventory = response.data.map(item => ({
             ...item,
             sellQuantity: item.totalQuantity,
+            // Vue3: besserPrices direkt als Property setzen (statt this.$set)
+            betterPrices: Array.isArray(item.betterPrices) ? item.betterPrices : [],
           }));
           await this.loadPricesForInventory();
         } catch (error) {
@@ -310,10 +312,11 @@
                 currentPrice: currentPrice
               }
             });
-            this.$set(item, 'betterPrices', data || []);
+            // Vue3: direkte Zuweisung ist reaktiv
+            item.betterPrices = Array.isArray(data) ? data : [];
           } catch (error) {
             console.error(`Error loading prices for item ${itemKey}:`, error);
-            this.$set(item, 'betterPrices', []);
+            item.betterPrices = [];
           } finally {
             this.loadingPrices.delete(itemKey);
           }