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/controllers/registrations_controller_test.rb

@@ -14,6 +14,35 @@ class RegistrationsControllerTest < ActionDispatch::IntegrationTest
     assert_redirected_to root_url
   end
 
+  test "first user of instance becomes super_admin" do
+    # Clear all users to simulate fresh instance
+    User.destroy_all
+
+    assert_difference "User.count", +1 do
+      post registration_url, params: { user: {
+        email: "firstuser@example.com",
+        password: "Password1!" } }
+    end
+
+    first_user = User.find_by(email: "firstuser@example.com")
+    assert first_user.super_admin?, "First user should be super_admin"
+  end
+
+  test "subsequent users become admin not super_admin" do
+    # Ensure users exist from fixtures
+    assert User.exists?
+
+    assert_difference "User.count", +1 do
+      post registration_url, params: { user: {
+        email: "seconduser@example.com",
+        password: "Password1!" } }
+    end
+
+    new_user = User.find_by(email: "seconduser@example.com")
+    assert new_user.admin?, "Subsequent user should be admin"
+    assert_not new_user.super_admin?, "Subsequent user should not be super_admin"
+  end
+
   test "create when hosted requires an invite code" do
     with_env_overrides REQUIRE_INVITE_CODE: "true" do
       assert_no_difference "User.count" do