fix(stringutil): use InvariantCulture for byte-size formatting

Locale-Bug: BytesToString rendert auf deutscher Locale "1,5GB" statt
"1.5GB". InvariantCulture pinnt den Dezimal-Separator. Plus
InternalsVisibleTo-Hook für ein lokales (gitignored) Test-Projekt.

HellionChat/HellionChat.csproj

@@ -36,6 +36,13 @@
         <PackageReference Include="SixLabors.ImageSharp" Version="[3.1.12, 4.0.0)" />
     </ItemGroup>
 
+    <ItemGroup>
+        <!-- Pure-function test suites in HellionChat.Tests need access to
+             the internal helper classes (StringUtil, UriPayload, Tokenizer
+             etc.). Test assembly does not get redistributed. -->
+        <InternalsVisibleTo Include="HellionChat.Tests" />
+    </ItemGroup>
+
     <ItemGroup>
         <Compile Update="Resources\Language.Designer.cs">
             <DesignTime>True</DesignTime>

HellionChat/Util/StringUtil.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.Text;
 
 namespace HellionChat.Util;
@@ -24,7 +25,8 @@ internal static class StringUtil
         var place = Convert.ToInt32(Math.Floor(Math.Log(bytes, 1024)));
         var num = Math.Round(bytes / Math.Pow(1024, place), 1);
         // "0.#" keeps the rounded fractional digit (1.5 GB stays "1.5GB"); "N0"
-        // would truncate it back to integer.
-        return (Math.Sign(byteCount) * num).ToString("0.#") + suf[place];
+        // would truncate it back to integer. InvariantCulture pins the decimal
+        // separator to '.' so a German locale doesn't render "1,5GB".
+        return (Math.Sign(byteCount) * num).ToString("0.#", CultureInfo.InvariantCulture) + suf[place];
     }
 }