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>

config/themes.php

@@ -0,0 +1,39 @@
+<?php
+
+return [
+    'default' => [
+        'name'    => 'Blau (Standard)',
+        'primary' => '#1d4ed8',
+        'dark'    => '#1e40af',
+        'light'   => '#eff6ff',
+        'text'    => '#1e40af',
+    ],
+    'green' => [
+        'name'    => 'Grün',
+        'primary' => '#16a34a',
+        'dark'    => '#15803d',
+        'light'   => '#f0fdf4',
+        'text'    => '#15803d',
+    ],
+    'red' => [
+        'name'    => 'Rot',
+        'primary' => '#dc2626',
+        'dark'    => '#b91c1c',
+        'light'   => '#fef2f2',
+        'text'    => '#b91c1c',
+    ],
+    'purple' => [
+        'name'    => 'Lila',
+        'primary' => '#7c3aed',
+        'dark'    => '#6d28d9',
+        'light'   => '#f5f3ff',
+        'text'    => '#6d28d9',
+    ],
+    'orange' => [
+        'name'    => 'Orange',
+        'primary' => '#ea580c',
+        'dark'    => '#c2410c',
+        'light'   => '#fff7ed',
+        'text'    => '#c2410c',
+    ],
+];