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>

database/migrations/2024_01_01_000017_create_email_templates_table.php

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+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration {
+    public function up(): void
+    {
+        Schema::create('email_templates', function (Blueprint $table) {
+            $table->id();
+            $table->string('key')->unique();
+            $table->string('label');
+            $table->string('subject');
+            $table->longText('body');
+            $table->json('variables')->nullable();
+            $table->timestamps();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('email_templates');
+    }
+};

database/migrations/2024_01_01_000018_add_layout_to_pages.php

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+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration {
+    public function up(): void
+    {
+        Schema::table('pages', function (Blueprint $table) {
+            $table->string('layout')->default('default')->after('show_in_footer');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('pages', function (Blueprint $table) {
+            $table->dropColumn('layout');
+        });
+    }
+};