Add email templates, page layouts, and shop themes (template system)

- E-Mail-Templates: DB-gespeicherte HTML-Templates mit {{placeholder}}-System,
  Admin-UI zum Bearbeiten von Betreff und Body, alle 4 Mailables aktualisiert
- Seiten-Layouts: 4 Layouts für CMS-Seiten (Standard, Sidebar, Volle Breite, Landing),
  Layout-Auswahl im Admin-Formular
- Shop-Themes: 5 Farbthemen via CSS-Variablen, Picker in Einstellungen
- Deployment: EmailTemplateSeeder wird automatisch beim Deploy ausgeführt

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

app/Mail/WiderrufsformularMail.php

@@ -2,6 +2,7 @@
 
 namespace App\Mail;
 
+use App\Models\EmailTemplate;
 use App\Models\Setting;
 use Illuminate\Bus\Queueable;
 use Illuminate\Mail\Mailable;
@@ -18,10 +19,32 @@ class WiderrufsformularMail extends Mailable
 
     public function build()
     {
-        $subject = $this->isConfirmation
-            ? 'Bestätigung Ihres Widerrufs – ' . Setting::get('shop_name')
-            : 'Neues Widerrufsformular eingegangen';
+        if (!$this->isConfirmation) {
+            $subject = 'Neues Widerrufsformular eingegangen';
+            return $this->subject($subject)->view('emails.widerruf');
+        }
 
-        return $this->subject($subject)->view('emails.widerruf');
+        $template = EmailTemplate::getByKey('widerruf_confirmation');
+
+        if (!$template) {
+            return $this
+                ->subject('Bestätigung Ihres Widerrufs – ' . Setting::get('shop_name'))
+                ->view('emails.widerruf');
+        }
+
+        $vars = [
+            'customer_name'          => $this->data['name'],
+            'widerruf_name'          => $this->data['name'],
+            'widerruf_email'         => $this->data['email'],
+            'widerruf_order_number'  => $this->data['order_number'] ?? '—',
+            'widerruf_product'       => $this->data['product'],
+            'widerruf_date'          => $this->data['date'],
+            'widerruf_reason'        => $this->data['reason'] ?? '—',
+            'shop_name'              => Setting::get('shop_name'),
+        ];
+
+        return $this
+            ->subject($template->renderSubject($vars))
+            ->html($template->renderBody($vars));
     }
 }