Make first user of instance automatically super_admin (#655)

* Implement dynamic role assignment for new family creators.

Introduced `User.role_for_new_family_creator` to assign `super_admin` to the first user of an instance and a configurable fallback role (e.g., `admin`) to subsequent users. Updated controllers and tests accordingly.

* Update default fallback role for family creators to admin.

---------

Co-authored-by: luckyPipewrench <luckypipewrench@proton.me>

test/models/user_test.rb

@@ -331,4 +331,21 @@ class UserTest < ActiveSupport::TestCase
     assert_not user.valid?
     assert_includes user.errors[:password], "can't be blank"
   end
+
+  # First user role assignment tests
+  test "role_for_new_family_creator returns super_admin when no users exist" do
+    # Delete all users to simulate fresh instance
+    User.destroy_all
+
+    assert_equal :super_admin, User.role_for_new_family_creator
+  end
+
+  test "role_for_new_family_creator returns fallback role when users exist" do
+    # Users exist from fixtures
+    assert User.exists?
+
+    assert_equal :admin, User.role_for_new_family_creator
+    assert_equal :member, User.role_for_new_family_creator(fallback_role: :member)
+    assert_equal "custom_role", User.role_for_new_family_creator(fallback_role: "custom_role")
+  end
 end