Hide contribution payments from demo user(s) (#738)

* Hide payment contribution options from demo and manually created users

Demo data users and manually created users don't have stripe_customer_id
set on their family, so they should not see payment/contribution options.

Changes:
- Add can_manage_subscription? method to Family::Subscribeable that checks
  for presence of stripe_customer_id
- Guard Settings::PaymentsController to return 403 for users without
  stripe_customer_id
- Guard SubscriptionsController#show action (Stripe portal redirect) for
  users without stripe_customer_id
- Update settings navigation to hide the payment link when
  stripe_customer_id is not present
- Add tests for the new behavior

* Fix broken test

---------

Co-authored-by: Claude <noreply@anthropic.com>

test/controllers/settings/payments_controller_test.rb

@@ -1,7 +1,22 @@
 require "test_helper"
 
 class Settings::PaymentsControllerTest < ActionDispatch::IntegrationTest
-  # test "the truth" do
-  #   assert true
-  # end
+  setup do
+    sign_in @user = users(:empty)
+    @family = @user.family
+  end
+
+  test "returns forbidden when family has no stripe_customer_id" do
+    assert_nil @family.stripe_customer_id
+
+    get settings_payment_path
+    assert_response :forbidden
+  end
+
+  test "shows payment settings when family has stripe_customer_id" do
+    @family.update!(stripe_customer_id: "cus_test123")
+
+    get settings_payment_path
+    assert_response :success
+  end
 end