fix: restart relaunches correctly when the server path contains a space (#2704)

`Core.DoKill(true)` built the relaunch command line by putting the assembly path into a raw argument string without quotes, so `dotnet` split it at the first space. That breaks `restart`, `AutoRestart` and `CrashGuard` recovery for any server installed under a path with a space.

- Pass the assembly path through `ProcessStartInfo.ArgumentList`, which quotes it.
- Relaunch through the host that started the process (`Environment.ProcessPath`): the `ModernUO` apphost runs directly, and `dotnet` gets the assembly path. Before this, a server started from the apphost came back under `dotnet`.
- Use one code path on all platforms. `UseShellExecute = true` on Unix made no difference for a program found on PATH.

This doesn't fix the second half of #2700, where the relaunched server loses the console (and gets killed under systemd or Docker). That comes from the server spawning a copy of itself and exiting, and a follow-up PR will add an opt-in restart exit code for a supervisor to act on.

**Testing:** relaunched from a directory with spaces, started both through `dotnet x.dll` and through the apphost, on Windows. The old code fails there with the error from the issue.

Projects/Server/Main.cs

@@ -473,22 +473,17 @@ public static class Core
             try
             {
                 logger.Information("Restarting");
-                if (IsWindows)
-                {
-                    using var process = Process.Start("dotnet", $"{ApplicationAssembly.Location}");
-                }
-                else
-                {
-                    using var process = new Process();
-                    process.StartInfo = new ProcessStartInfo
-                    {
-                        FileName = "dotnet",
-                        Arguments = $"{ApplicationAssembly.Location}",
-                        UseShellExecute = true
-                    };
 
-                    process.Start();
+                // Relaunch through whichever host started us: the apphost (ModernUO.exe) runs directly,
+                // `dotnet ModernUO.dll` needs the assembly path. ArgumentList quotes paths with spaces.
+                var hostPath = Environment.ProcessPath ?? "dotnet";
+                var startInfo = new ProcessStartInfo(hostPath) { UseShellExecute = false };
+                if (Path.GetFileNameWithoutExtension(hostPath).Equals("dotnet", StringComparison.OrdinalIgnoreCase))
+                {
+                    startInfo.ArgumentList.Add(ApplicationAssembly.Location);
                 }
+
+                using var process = Process.Start(startInfo);
                 logger.Information("Restart done");
             }
             catch (Exception e)