Fix controller tests and SQLite compatibility for defaults_diff

Test infrastructure:
- Allow www.example.com host in test env (ActionDispatch::HostAuthorization
  was blocking all integration test requests)
- Include Devise::Test::IntegrationHelpers in ActionDispatch::IntegrationTest
  so tests can sign in with sign_in(user)

Controller tests:
- Rewrite UsersControllerTest to match actual routes/actions (no new/create/
  edit/destroy); sign in as admin; test update-self rejection via turbo_stream
- Fix Default::UnitsControllerTest to sign in before requesting the index

SQLite compatibility in Unit#defaults_diff:
- Hoist the inner "units" CTE to the outer WITH RECURSIVE level (fixes nested
  WITH syntax error) — this was the existing TODO in the code
- Use Unit.joins(...) for the recursive part instead of a raw Arel::SelectManager
  so the SQLite visitor does not wrap it in parentheses inside UNION ALL
- Drop the named "units" CTE (conflicts with the table name under WITH RECURSIVE
  in SQLite); apply the user/defaults scope directly on the base case
- Qualify GROUP BY columns to avoid ambiguity when bases_units is joined
- Qualify ORDER BY :multiplier/:symbol to avoid ambiguity (Unit.ordering)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/test_helper.rb

@@ -2,6 +2,10 @@ ENV["RAILS_ENV"] ||= "test"
 require_relative "../config/environment"
 require "rails/test_help"
 
+class ActionDispatch::IntegrationTest
+  include Devise::Test::IntegrationHelpers
+end
+
 class ActiveSupport::TestCase
   # Run tests in parallel with specified workers
   parallelize(workers: :number_of_processors)