fix(loyalty): use confirm modal for category deletion

Replace browser confirm() dialog with the shared confirm_modal
macro for category deletion. Matches the existing program delete
pattern. Shows warning about impact on existing transactions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/static/admin/js/loyalty-merchant-detail.js

@@ -41,6 +41,8 @@ function adminLoyaltyMerchantDetail() {
         newCategoryTranslations: { fr: '', de: '', lb: '' },
         viewingCategoryId: null,
         editingCategoryId: null,
+        showDeleteCategoryModal: false,
+        categoryToDelete: null,
         editCategoryData: { name: '', translations: { fr: '', de: '', lb: '' } },
 
         // State
@@ -353,14 +355,17 @@ function adminLoyaltyMerchantDetail() {
             }
         },
 
-        async deleteCategory(catId) {
-            if (!confirm('Delete this category?')) return;
+        async confirmDeleteCategory() {
+            if (!this.categoryToDelete) return;
             try {
-                await apiClient.delete(`/admin/loyalty/stores/${this.selectedCategoryStoreId}/categories/${catId}`);
+                await apiClient.delete(`/admin/loyalty/stores/${this.selectedCategoryStoreId}/categories/${this.categoryToDelete}`);
                 await this.loadCategoriesForStore();
                 Utils.showToast('Category deleted', 'success');
             } catch (error) {
                 Utils.showToast(error.message || 'Failed to delete category', 'error');
+            } finally {
+                this.showDeleteCategoryModal = false;
+                this.categoryToDelete = null;
             }
         }
     };