feat(desec): keep the typed slug across the registration flow

A slug typed into the deSEC form was lost when the form re-rendered after a
201 redirect (e.g. on the awaiting-verification step), forcing the user to
re-enter it. Persist the requested slug in the AIO config when register()
runs and pre-fill the slug input from it on every step, then clear it once a
domain is set. Stored in configuration.json rather than a URL parameter so it
survives the multi-step flow without leaking into the iframe address.

Signed-off-by: Simon L. <szaimen@e.mail.de>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

php/src/Data/ConfigurationManager.php

@@ -213,6 +213,16 @@ class ConfigurationManager
         set { $this->set('desec_email', $value); }
     }
 
+    /**
+     * The subdomain slug the user last requested. Persisted across the multi-step
+     * registration flow so the slug input can be pre-filled when the form re-renders
+     * (e.g. after email verification). Not a secret; cleared once a domain is set.
+     */
+    public string $desecSlug {
+        get => $this->get('desec_slug', '');
+        set { $this->set('desec_slug', $value); }
+    }
+
     /**
      * Base URL of the deSEC API. Configurable via the 'desec_api_base' config key
      * (configuration.json) only — intentionally NOT an environment variable — so the