net8.0: upgrade from netcoreapp3.1, fix Linux rendering

- TFM: netcoreapp3.1 → net8.0, netstandard2.1 → net8.0
- System.Drawing.Common 5.0.3 → 6.0.0 (last version with Unix support)
- Remove stale packages (NETStandard.Library, System.Collections)
- Set System.Drawing.EnableUnixSupport via runtimeconfig + AppContext
- Pin System.Resources.Extensions 8.0.0, suppress NU1603
- Copy game_folder assets to build output (no run.sh required)
- Sta.CurrentDirectory → AppContext.BaseDirectory for portable path resolution
- Enable UnsafeBinaryFormatterSerialization + InvariantGlobalization
- Add global.json pinning SDK to 8.x
- Remove ambiguous `using System.Net.Mime.MediaTypeNames` in ParT.cs
- Add `using System.Reflection` to Program.cs

2DGAMELIB/_2DGAMELIB/ParT.cs

@@ -2,7 +2,6 @@ using Newtonsoft.Json;
 using System;
 using System.Drawing;
 using System.Drawing.Drawing2D;
-using static System.Net.Mime.MediaTypeNames;
 
 namespace _2DGAMELIB
 {