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.Core/ConversionOptions.cs

@@ -0,0 +1,21 @@
+namespace ConvertSRTto3DASS;
+
+public class ConversionOptions
+{
+    public string InputPath { get; set; } = "";
+    public string OutputPath { get; set; } = "";
+
+    public string Mode { get; set; } = "sbs"; // sbs, ou, rg
+
+    public int ResX { get; set; } = 1280;
+    public int ResY { get; set; } = 720;
+    public int BaseResX { get; set; } = 1280;
+    public int BaseResY { get; set; } = 720;
+
+    public int FontSize { get; set; } = 16;
+    public int OffsetX { get; set; } = 4;
+    public int BottomOffset { get; set; } = 18;
+    public int SbsSideMargin { get; set; } = 640;
+    public int OuTopMargin { get; set; } = 385;
+    public int VerticalMargin { get; set; } = 25;
+}