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/controllers/splits_controller_test.rb

@@ -17,6 +17,21 @@ class SplitsControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
   end
 
+  test "new groups subcategories immediately after their parent in the category selector" do
+    get new_transaction_split_path(@entry)
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    category_values = doc.css(".category-select-container [data-value]").map { |node| node["data-value"] }
+
+    parent_index = category_values.index(categories(:food_and_drink).id)
+    child_index = category_values.index(categories(:subcategory).id)
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
   test "create with valid params splits transaction" do
     assert_difference "Entry.count", 2 do
       post transaction_split_path(@entry), params: {