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/Http/Controllers/Admin/EmailTemplateController.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Http\Controllers\Admin;
+
+use App\Http\Controllers\Controller;
+use App\Models\EmailTemplate;
+use Illuminate\Http\Request;
+
+class EmailTemplateController extends Controller
+{
+    public function index()
+    {
+        $templates = EmailTemplate::orderBy('label')->get();
+        return view('admin.email-templates.index', compact('templates'));
+    }
+
+    public function edit(EmailTemplate $emailTemplate)
+    {
+        return view('admin.email-templates.edit', ['template' => $emailTemplate]);
+    }
+
+    public function update(Request $request, EmailTemplate $emailTemplate)
+    {
+        $request->validate([
+            'subject' => 'required|string|max:255',
+            'body'    => 'required|string',
+        ]);
+
+        $emailTemplate->update([
+            'subject' => $request->input('subject'),
+            'body'    => $request->input('body'),
+        ]);
+
+        return back()->with('success', 'E-Mail-Template gespeichert.');
+    }
+}