feat: modernize to .NET 8, split into Core/Cli/Gui (#3)

* feat: modernize to .NET 8, split into Core/Cli/Gui projects

Core: shared converter library (cross-platform)

Cli: console app with CLI args (cross-platform)

Gui: WinForms app (Windows only, uses Core library)

- Split into 3 projects sharing ConvertSRTto3DASS.Core

- Rewrite SrtConverter with clean public API

- Add CLI entry point with stdlib argument parsing

- Redesign GUI with GroupBox layout, flat buttons, modern fonts

- Update README with publish commands

* feat: add drag-and-drop to GUI

* refactor: rename GUI controls to proper PascalCase identifiers

---------

Co-authored-by: imrayya <imrayya@users.noreply.github.com>

ConvertSRTto3DASS.Gui/Properties/Settings.Designer.cs

@@ -0,0 +1,30 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by a tool.
+//     Runtime Version:4.0.30319.42000
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace ConvertSRTto3DASS.Properties
+{
+
+
+    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.VisualStudio.Editors.SettingsDesigner.SettingsSingleFileGenerator", "11.0.0.0")]
+    internal sealed partial class Settings : global::System.Configuration.ApplicationSettingsBase
+    {
+
+        private static Settings defaultInstance = ((Settings)(global::System.Configuration.ApplicationSettingsBase.Synchronized(new Settings())));
+
+        public static Settings Default
+        {
+            get
+            {
+                return defaultInstance;
+            }
+        }
+    }
+}