Ensure the admin sign in root path fallback is '/admin'

Previously relied on solidus_frontend defining `spree.root_path`, and
used it redirecting admin users to the store home page.

Author: elia

lib/controllers/backend/spree/admin/user_sessions_controller.rb

@@ -35,6 +35,10 @@ def authorization_failure
 
   private
 
+  def signed_in_root_path(_resource)
+    spree.admin_path
+  end
+
   # NOTE: as soon as this gem stops supporting Solidus 3.1 if-else should be removed and left only include
   if defined?(::Spree::Admin::SetsUserLanguageLocaleKey)
     include ::Spree::Admin::SetsUserLanguageLocaleKey

spec/controllers/spree/admin/user_sessions_controller_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+RSpec.describe Spree::Admin::UserSessionsController, type: :controller do
+  let(:user) { create(:user, password: 'secret') }
+
+  before { @request.env['devise.mapping'] = Devise.mappings[:spree_user] }
+
+  it "redirects to the admin root after signing in with no stored location" do
+    get :new
+
+    post(:create,
+      params: {
+        spree_user: {
+          email: user.email,
+          password: 'secret'
+        },
+      }
+    )
+
+    expect(response).to redirect_to spree.admin_path
+  end
+end

spec/features/admin/sign_in_spec.rb

@@ -2,7 +2,7 @@
 
 RSpec.feature 'Admin - Sign In', type: :feature do
   background do
-    @user = create(:user, email: 'email@person.com')
+    @user = create(:admin_user, email: 'email@person.com')
     visit spree.admin_login_path
   end
 
@@ -19,7 +19,7 @@
     expect(page).to have_text 'Logged in successfully'
     expect(page).not_to have_text 'Login'
     expect(page).to have_text 'Logout'
-    expect(current_path).to eq '/'
+    expect(current_path).to eq '/admin/orders'
   end
 
   scenario 'shows validation erros' do