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

SlaveMatrix/Properties/launchSettings.json

@@ -2,7 +2,10 @@
   "profiles": {
     "SlaveMatrix": {
       "commandName": "Project",
-      "workingDirectory": "$(SolutionDir)\\game_folder"
+      "workingDirectory": "../game_folder",
+      "environmentVariables": {
+        "DOTNET_SYSTEM_GLOBALIZATION_INVARIANT": "true"
+      }
     },
     "WSL": {
       "commandName": "WSL2",