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/PaymentReceivedMail.php

@@ -2,6 +2,7 @@
 
 namespace App\Mail;
 
+use App\Models\EmailTemplate;
 use App\Models\Order;
 use App\Models\Setting;
 use Illuminate\Bus\Queueable;
@@ -20,9 +21,29 @@ class PaymentReceivedMail extends Mailable
 
     public function build()
     {
+        $order    = $this->order;
+        $template = EmailTemplate::getByKey('payment_received');
+
+        if (!$template) {
+            return $this
+                ->subject('Ihre Rechnung – ' . $order->order_number . ' – ' . Setting::get('shop_name'))
+                ->view('emails.payment_received')
+                ->attachData($this->pdfContent, $this->pdfFilename, ['mime' => 'application/pdf']);
+        }
+
+        $customerName = $order->billingAddress()?->full_name ?? $order->guest_email;
+
+        $data = [
+            'customer_name' => $customerName,
+            'order_number'  => $order->order_number,
+            'order_total'   => number_format($order->total, 2, ',', '.'),
+            'shop_name'     => Setting::get('shop_name'),
+            'shop_email'    => Setting::get('shop_email'),
+        ];
+
         return $this
-            ->subject('Ihre Rechnung – ' . $this->order->order_number . ' – ' . Setting::get('shop_name'))
-            ->view('emails.payment_received')
+            ->subject($template->renderSubject($data))
+            ->html($template->renderBody($data))
             ->attachData($this->pdfContent, $this->pdfFilename, ['mime' => 'application/pdf']);
     }
 }