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/transactions_controller_test.rb

@@ -8,6 +8,21 @@ class TransactionsControllerTest < ActionDispatch::IntegrationTest
     @entry = entries(:transaction)
   end
 
+  test "index groups subcategories immediately after their parent in the category filter" do
+    get transactions_url
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    checkbox_values = doc.css("input[name='q[categories][]']").map { |node| node["value"] }
+
+    parent_index = checkbox_values.index(categories(:food_and_drink).name)
+    child_index = checkbox_values.index(categories(:subcategory).name)
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
   test "creates with transaction details" do
     assert_difference [ "Entry.count", "Transaction.count" ], 1 do
       post transactions_url, params: {
@@ -467,6 +482,37 @@ end
     assert_not entry.protected_from_sync?
   end
 
+  test "new groups subcategories immediately after their parent in the category select" do
+    get new_transaction_url
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    trigger = doc.at_css("#category_id_trigger")
+    assert_not_nil trigger, "expected the category select trigger button to render"
+
+    wrapper = trigger.ancestors(".relative").first
+    category_values = wrapper.css("[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 "new renders a search box for account selection" do
+    get new_transaction_url
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    trigger = doc.at_css("#account_id_trigger")
+    assert_not_nil trigger, "expected the account select trigger button to render"
+
+    wrapper = trigger.ancestors(".relative").first
+    assert_not_nil wrapper.at_css("input[type='search']"), "expected a search input inside the account select"
+  end
+
   test "new with duplicate_entry_id pre-fills form from source transaction" do
     @entry.reload