Add test coverage for category hierarchy and account search

Model-level:
- Category::GroupTest (new): for() grouping and the new select_options
  helper (order + indent labels).
- CategoryTest: alphabetically_by_hierarchy scope ordering.
- Rule::ConditionFilter::TransactionCategoryTest (new)
- Rule::ActionExecutor::SetTransactionCategoryTest (new)
- Import::CategoryMappingTest (new): grouping + 'Add as new category'
  still prepends correctly.

Controller/integration-level (asserting actual rendered HTML order):
- SplitsControllerTest: category combobox data-value ordering.
- Transactions::CategorizesControllerTest: bulk-categorize <select>
  option ordering.
- TransactionsControllerTest:
  - search filter checkbox ordering (q[categories][])
  - new-transaction DS::Select category ordering (via trigger id +
    ancestor traversal)
  - new-transaction account select renders a search box

All new/modified test files verified with 'ruby -c' (syntax) and
cross-checked fixture names, family scoping, route helpers, and field
names against the actual fixtures/routes/views. Ruby/Bundler network
access to rubygems.org is unavailable in this sandbox, so the suite
itself has not been executed — run 'bin/rails test' before merging.

test/models/category_test.rb

@@ -159,4 +159,15 @@ class CategoryTest < ActiveSupport::TestCase
     assert lookup.key?(category.id)
     assert_not lookup.key?(0)
   end
+
+  test "alphabetically_by_hierarchy orders a category's subcategories immediately after it" do
+    ordered = @family.categories.alphabetically_by_hierarchy.to_a
+
+    parent_index = ordered.index(categories(:food_and_drink))
+    child_index = ordered.index(categories(:subcategory))
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
 end