feat: implement two-step category and product picker with private product creation support

flutter/lib/core/ui/category_then_product_picker.dart

@@ -73,7 +73,7 @@ class CategoryThenProductPicker {
     }
 
     // Samla alla kategori-IDs i den valda grenen (inkl. ättlingar)
-    final categoryIds = _collectIds(selectedCategory);
+    final categoryIds = _collectIds(selectedCategory!);
 
     // Filtrera produkter på dessa kategorier
     final filtered = products
@@ -84,7 +84,7 @@ class CategoryThenProductPicker {
     // Bygg eventuell onCreate-callback med categoryId inbunden
     final onCreateBound = onCreate == null
         ? null
-        : (String name) => onCreate(name, selectedCategory.id);
+        : (String name) => onCreate(name, selectedCategory!.id);
 
     // Steg 2 — välj produkt
     if (!context.mounted) return null;
@@ -92,7 +92,7 @@ class CategoryThenProductPicker {
       context,
       products: useList,
       value: currentProductId,
-      label: 'Produkt i "${selectedCategory.name}"',
+      label: 'Produkt i "${selectedCategory!.name}"',
       categoryFilter: null, // redan förfiltrerat
       onCreate: onCreateBound,
     );