Offer «Entrar con Pireon» on the panel's login page

Under the e-mail and password form, a button takes the redirect flow
(Authorization Code + PKCE), so the password is only ever typed at the
identity server. The routes for it were already there; the page had no
way to reach them.

A Pireon client allows one grant type and the panel's is a password
client, so the redirect flow can use its own: PIREON_REDIRECT_CLIENT_ID
and PIREON_REDIRECT_CLIENT_SECRET (the main client when they are empty).
The button shows up once that client is set, or with
PIREON_REDIRECT_LOGIN=true. Each session renews its token with the
client that issued it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lang/en/pireon.php

@@ -4,7 +4,8 @@ declare(strict_types=1);
 
 return [
     'sign_in' => 'Sign in',
-    'sign_in_intro' => 'Sign in with your e-mail and password.',
+    'sign_in_intro' => 'Sign in with your e-mail and password, or with your Pireon account.',
+    'continue_with_pireon' => 'Sign in with Pireon',
     'signing_in' => 'Checking your credentials…',
     'password_grant_disabled' => 'Password sign-in is switched off on this installation. Please tell an administrator.',
     'server_unavailable' => 'The identity server is not responding. This is not about your credentials; please tell an administrator.',