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

global.json

@@ -0,0 +1,7 @@
+{
+  "sdk": {
+    "version": "8.0.0",
+    "rollForward": "latestMajor",
+    "allowPrerelease": true
+  }
+}