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/OrderShippedMail.php

@@ -2,6 +2,7 @@
 
 namespace App\Mail;
 
+use App\Models\EmailTemplate;
 use App\Models\Order;
 use App\Models\Setting;
 use Illuminate\Bus\Queueable;
@@ -16,8 +17,32 @@ class OrderShippedMail extends Mailable
 
     public function build()
     {
+        $order    = $this->order;
+        $template = EmailTemplate::getByKey('order_shipped');
+
+        if (!$template) {
+            return $this
+                ->subject('Ihre Bestellung wurde versendet – ' . $order->order_number)
+                ->view('emails.order_shipped');
+        }
+
+        $trackingInfo = $order->tracking_code
+            ? '<div style="background:#f0fdf4;padding:16px;border-radius:8px;border:1px solid #86efac;margin:16px 0">'
+              . '<strong>Sendungsverfolgung:</strong><br>'
+              . 'Tracking-Nummer: <strong>' . e($order->tracking_code) . '</strong>'
+              . '</div>'
+            : '';
+
+        $data = [
+            'order_number'  => $order->order_number,
+            'tracking_code' => $order->tracking_code ?? '',
+            'tracking_info' => $trackingInfo,
+            'shop_name'     => Setting::get('shop_name'),
+            'shop_email'    => Setting::get('shop_email'),
+        ];
+
         return $this
-            ->subject('Ihre Bestellung wurde versendet – ' . $this->order->order_number)
-            ->view('emails.order_shipped');
+            ->subject($template->renderSubject($data))
+            ->html($template->renderBody($data));
     }
 }