Add CSS hook, URL ref prefill, order prefill, and Widerrufsfrist display

- Fix CSS loading: register stylesheet via actionFrontControllerSetMedia
  hook instead of initContent() — guaranteed to fire before page render
- Upgrade script registers new hook on existing installations
- Feature 1: pre-fill order_reference from ?ref= URL parameter
- Feature 2: pre-fill customer name/email from logged-in account when
  navigating with ?ref= (already worked; now order ref is also pre-filled)
- Feature 4: compute Widerrufsfrist (order date +14 days) from DB and
  display on form (when ref in URL), confirm, and success pages
- Feature 3: EN mail templates were already present

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

upgrade/upgrade-0.1.5.php

@@ -0,0 +1,10 @@
+<?php
+
+if (!defined('_PS_VERSION_')) {
+    exit;
+}
+
+function upgrade_module_0_1_5($object)
+{
+    return $object->registerHook('actionFrontControllerSetMedia');
+}