feat: clean up imports and fix minor formatting issues in various files

flutter/lib/core/ui/app_shell.dart

@@ -1,6 +1,5 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
-import 'package:go_router/go_router.dart';
 
 import '../../features/auth/data/auth_providers.dart';
 import '../../features/recipes/data/recipes_grid_provider.dart';

flutter/lib/core/ui/category_then_product_picker.dart

@@ -35,8 +35,6 @@ class CategoryThenProductPicker {
     }
     return ids;
   }
-    return null;
-  }
 
   /// Öppnar ett bottenark med kategoriträdet. Klick på en lövnod (inga barn)
   /// öppnar direkt produktpickern för den kategorin.

flutter/lib/core/ui/product_picker_field.dart

@@ -105,8 +105,7 @@ class ProductPickerField extends StatelessWidget {
       onChanged?.call(null);
       return;
     }
-    if (result is! int) return;
-    onChanged?.call(result);
+    onChanged?.call(result as int);
   }
 
   /// Öppnar produktväljarens bottenark utan att binda den till en specifik widget-instans.