feat: add external drawing boxes and paged callbacks

LehrerApp.Templating/DrawingFontRegistry.cs

@@ -0,0 +1,29 @@
+using System.Security.Cryptography;
+using QuestPDF.Drawing;
+
+namespace LehrerApp.Templating;
+
+/// <summary>
+/// Registers application-supplied TTF/OTF font bytes for TEXT and drawing commands without
+/// exposing QuestPDF types to the calling application.
+/// </summary>
+public static class DrawingFontRegistry
+{
+    private static readonly object Gate = new();
+    private static readonly HashSet<string> Registered = new(StringComparer.Ordinal);
+
+    public static void RegisterFont(string familyName, byte[] fontData)
+    {
+        if (string.IsNullOrWhiteSpace(familyName) || familyName.Length > 80)
+            throw new ArgumentException("Der Schriftname fehlt oder ist zu lang.", nameof(familyName));
+        if (fontData.Length is 0 or > 20 * 1024 * 1024)
+            throw new ArgumentException("Eine Schriftdatei muss zwischen 1 Byte und 20 MB groß sein.", nameof(fontData));
+        var key = familyName + ":" + Convert.ToHexString(SHA256.HashData(fontData));
+        lock (Gate)
+        {
+            if (!Registered.Add(key)) return;
+            using var stream = new MemoryStream(fontData, writable: false);
+            FontManager.RegisterFontWithCustomName(familyName, stream);
+        }
+    }
+}