feat: add red-green 3D mode and Windows Forms GUI

- add rg (red-green) anaglyph subtitle mode with configurable horizontal offset
- extend CLI mode handling to support sbs, ou, and rg
- add offsetx option for anaglyph subtitle positioning
- add Windows Forms GUI project for configuring and running conversions
- add file pickers for input/output paths in the GUI
- add GUI controls for mode, resolution, font size, and offset
- add status output and mode-aware UI behavior in the GUI
- update mode selection handling to support user-friendly display labels

ConvertSRTto3DASSGUI/Program.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows.Forms;
+
+namespace ConvertSRTto3DASSGUI
+{
+    internal static class Program
+    {
+        /// <summary>
+        /// The main entry point for the application.
+        /// </summary>
+        [STAThread]
+        static void Main()
+        {
+            Application.EnableVisualStyles();
+            Application.SetCompatibleTextRenderingDefault(false);
+            Application.Run(new Form1());
+        }
+    }
+}