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/Program.cs

@@ -0,0 +1,11 @@
+namespace ConvertSRTto3DASS;
+
+internal static class Program
+{
+    [STAThread]
+    static void Main()
+    {
+        ApplicationConfiguration.Initialize();
+        Application.Run(new Form1());
+    }
+}