Server updates from the web panel with progress screen

- Updater checks git.mbm.wtf every 6 h (and on demand) for a newer <Version>
  on main, shows the new commits and the required mod version
- Install: download the source, build it with dotnet publish into
  data/app/current, warn players and restart (exit code 75 for Docker/systemd,
  self-relaunch on Windows). At startup a newer installed update is loaded in
  its own AssemblyLoadContext; after 3 failed starts the built-in version runs
- Panel: Update page, sidebar badge, full-screen progress overlay (download,
  build, install, restart) that survives the restart and reloads the panel
- Panel sessions survive restarts (hashed tokens in data/panel_sessions.json)
- Docker image is based on the .NET SDK so it can build updates
- Version 2.9.0

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ServerHost.cs

@@ -36,6 +36,10 @@ public sealed class ServerHost
     public ServerConfig Config { get; private set; }
     public GameServer Server { get; private set; }
     public AdminPanel Panel { get; private set; }
+    /// <summary>Server-Updates (Prüfen/Installieren im Panel).</summary>
+    public Updater Updater { get; }
+    /// <summary>Rückgabewert von Run - Updater.RestartExitCode nach einem installierten Update.</summary>
+    public int ExitCode { get; private set; }
     public HostState State { get; private set; } = HostState.Stopped;
     public string LastError { get; private set; }
     public DateTime StateSince { get; private set; } = DateTime.UtcNow;
@@ -50,6 +54,7 @@ public sealed class ServerHost
         this.dataDir = dataDir;
         this.configPath = configPath;
         this.offline = offline;
+        Updater = new Updater(this, dataDir);
     }
 
     // ---------------------------------------------------------------- Steuerung (Konsole, Panel, Signale)
@@ -120,6 +125,13 @@ public sealed class ServerHost
         return "Restarting - the server is back in a few seconds";
     }
 
+    /// <summary>Update installiert: Weltzustand speichern, beenden und neu starten (Docker/systemd bzw. selbst).</summary>
+    public void ExitForUpdate()
+    {
+        ExitCode = Updater.RestartExitCode;
+        RequestExit();
+    }
+
     /// <summary>Ganzes Programm beenden (Strg+C, docker stop, "exit").</summary>
     public void RequestExit()
     {
@@ -136,6 +148,7 @@ public sealed class ServerHost
         if (!Listen(Config.Port)) return 1;
         if (Panel == null)
             GameServer.Log("Web panel is disabled (PanelEnabled in server.json)");
+        if (!offline) Updater.StartBackground();
 
         while (!exitRequested)
         {
@@ -185,7 +198,7 @@ public sealed class ServerHost
         try { listener?.Stop(); } catch { }
         Panel?.Stop();
         GameServer.Log("Bye.");
-        return 0;
+        return ExitCode;
     }
 
     void HandleHostCommands()