Fix category merge form submission (#3225)

app/views/categories/merge.html.erb

@@ -23,6 +23,7 @@
 
       <%= render DS::Button.new(
         text: t(".submit"),
+        type: :submit,
         full_width: true
       ) %>
     <% end %>

test/controllers/categories_controller_test.rb

@@ -172,6 +172,7 @@ class CategoriesControllerTest < ActionDispatch::IntegrationTest
 
     assert_response :success
     assert_select "#mobile-settings-nav"
+    assert_select "form[action='#{perform_merge_categories_path}'] button[type='submit']"
   end
 
   test "merge renders without the settings layout for modal frame requests" do