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/ConvertSRTto3DASS.Gui.csproj

@@ -0,0 +1,44 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net8.0-windows</TargetFramework>
+    <UseWindowsForms>true</UseWindowsForms>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <Company>imrayya</Company>
+    <Copyright>Copyright © imrayya 2021-2026</Copyright>
+    <AssemblyName>ConvertSRTto3DASS</AssemblyName>
+    <RootNamespace>ConvertSRTto3DASS</RootNamespace>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(Configuration)' == 'Release'">
+    <Optimize>true</Optimize>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\ConvertSRTto3DASS.Core\ConvertSRTto3DASS.Core.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Update="Form1.Designer.cs">
+      <DependentUpon>Form1.cs</DependentUpon>
+    </Compile>
+    <Compile Update="Properties\Resources.Designer.cs">
+      <DesignTime>True</DesignTime>
+      <AutoGen>True</AutoGen>
+      <DependentUpon>Resources.resx</DependentUpon>
+    </Compile>
+  </ItemGroup>
+
+  <ItemGroup>
+    <EmbeddedResource Update="Form1.resx">
+      <DependentUpon>Form1.cs</DependentUpon>
+    </EmbeddedResource>
+    <EmbeddedResource Update="Properties\Resources.resx">
+      <Generator>ResXFileCodeGenerator</Generator>
+      <LastGenOutput>Resources.Designer.cs</LastGenOutput>
+    </EmbeddedResource>
+  </ItemGroup>
+
+</Project>