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/Properties/Settings.settings

@@ -0,0 +1,7 @@
+<?xml version='1.0' encoding='utf-8'?>
+<SettingsFile xmlns="http://schemas.microsoft.com/VisualStudio/2004/01/settings" CurrentProfile="(Default)">
+  <Profiles>
+    <Profile Name="(Default)" />
+  </Profiles>
+  <Settings />
+</SettingsFile>