Baseline: Ausgangszustand vor Modularisierung

Erster Commit des bestehenden monolithischen WinForms-Copytraders,
inklusive der Alt-Backups (*.bak), damit diese dauerhaft in der
Historie rekonstruierbar bleiben. Threema-Lib unter libs/ wurde
vendored (nested .git entfernt).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

libs/Threema-MsgApi-Net-Core/IcgSoftware.Threema.CoreMsgApi/Messages/ThreemaMessage.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace IcgSoftware.Threema.CoreMsgApi.Messages
+{
+	/// <summary>
+	/// Abstract base class of messages that can be sent with end-to-end encryption via Threema.
+	/// </summary>
+	public abstract class ThreemaMessage
+	{
+		public const int NONCEBYTES = 24;
+		public const int BLOB_ID_LEN = 16;
+
+		/// <summary>
+		/// Get message's raw content
+		/// </summary>
+		/// <returns></returns>
+		public abstract byte[] GetData();
+
+		/// <summary>
+		/// Get message's type code
+		/// </summary>
+		/// <returns></returns>
+		public abstract int GetTypeCode();
+	}
+}