feat(auth): add user invitation flow with account setup

- add `createAccountSetup`, `verifyAccountSetupToken`, `deleteAccountSetupToken` to verifications core
- add `completeAccountSetup` function to auth core for password creation on invite
- add `InvitationEmail` template for sending invite links
- add `SetupAccountPage` client page for invited users to set their password
- add `UserCreateModal` admin component to invite new users
- wire invitation action and API endpoint in auth feature
- update admin `UsersPage` to include user creation modal
- update auth and admin README docs

src/features/auth/AuthPage.client.js

@@ -8,6 +8,7 @@ import ForgotPasswordPage from './pages/ForgotPasswordPage.client.js';
 import ResetPasswordPage from './pages/ResetPasswordPage.client.js';
 import ConfirmEmailPage from './pages/ConfirmEmailPage.client.js';
 import LogoutPage from './pages/LogoutPage.client.js';
+import SetupAccountPage from './pages/SetupAccountPage.client.js';
 
 const PAGE_COMPONENTS = {
   login:    LoginPage,
@@ -16,6 +17,7 @@ const PAGE_COMPONENTS = {
   reset:    ResetPasswordPage,
   confirm:  ConfirmEmailPage,
   logout:   LogoutPage,
+  setup:    SetupAccountPage,
 };
 
 export default function AuthPage({
@@ -26,6 +28,7 @@ export default function AuthPage({
   forgotPasswordAction,
   resetPasswordAction,
   verifyEmailAction,
+  setupAccountAction,
   logoutAction,
   setSessionCookieAction,
   redirectAfterLogin = '/',
@@ -81,6 +84,8 @@ export default function AuthPage({
       return <Page {...common} onSubmit={resetPasswordAction} email={email} token={token} />;
     case ConfirmEmailPage:
       return <Page {...common} onSubmit={verifyEmailAction} email={email} token={token} />;
+    case SetupAccountPage:
+      return <Page {...common} onSubmit={setupAccountAction} email={email} token={token} />;
     case LogoutPage:
       return <Page onLogout={logoutAction} onSetSessionCookie={setSessionCookieAction} />;
     default: