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>

routes/web.php

@@ -14,8 +14,9 @@ Route::redirect('/', '/admin');
 | Authentication — Pireon OAuth2 (Authorization Code + PKCE)
 |--------------------------------------------------------------------------
 |
-| `login` is the page Filament bounces guests to; it renders a single button
-| that starts the flow at `auth.redirect`.
+| `login` is the page Filament bounces guests to: an e-mail and password form
+| (password grant) with «Entrar con Pireon» under it, which starts this flow at
+| `auth.redirect`.
 |
 */