Refaktorierung für HTML-Sanitization mit neuer Sanitizer-Klasse.

- Bestehende Sanitization überarbeitet.
- Injektionsmethode für flexible Konfiguration implementiert.
- Wichtige Abschnitte in `show-envelope` hervorgehoben.

EnvelopeGenerator.Web/Sanitizers/DIExtensions.cs

@@ -0,0 +1,17 @@
+using Ganss.Xss;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace EnvelopeGenerator.Web.Sanitizers
+{
+    public static class DIExtensions
+    {
+        public static IServiceCollection AddSanitizer<THtmlSanitizer>(this IServiceCollection services, Action<THtmlSanitizer>? optionActions = null)
+            where THtmlSanitizer : HtmlSanitizer => services
+            .AddSingleton(serviceProvider =>
+            {
+                var sanitizer = ActivatorUtilities.CreateInstance<THtmlSanitizer>(serviceProvider);
+                optionActions?.Invoke(sanitizer);
+                return sanitizer;
+            });
+    }
+}

EnvelopeGenerator.Web/Sanitizers/HighlightHtmlSanitizer.cs

@@ -0,0 +1,8 @@
+using Ganss.Xss;
+
+namespace EnvelopeGenerator.Web.Sanitizers
+{
+    public class HighlightHtmlSanitizer : HtmlSanitizer
+    {
+    }
+}