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>

Dockerfile

@@ -8,13 +8,15 @@ WORKDIR /src
 COPY . .
 RUN dotnet publish -c Release -o /app
 
-# aspnet: enthält Kestrel für das Web-Panel
-FROM mcr.microsoft.com/dotnet/aspnet:10.0
+# sdk (enthält auch ASP.NET/Kestrel fürs Web-Panel): damit sich der Server im Panel selbst updaten kann
+# (lädt den Quellcode, baut ihn und legt ihn nach /data/app)
+FROM mcr.microsoft.com/dotnet/sdk:10.0
 WORKDIR /app
 COPY --from=build /app .
 ENV MSCO_DATA=/data
 # läuft als beliebige Benutzer-ID (docker compose "user:") - dann gibt es kein eigenes Home
 ENV HOME=/tmp
+ENV DOTNET_CLI_TELEMETRY_OPTOUT=1 DOTNET_NOLOGO=1
 # Befehle an den laufenden Server:  docker exec msco-server msco players
 RUN printf '#!/bin/sh\nexec dotnet /app/MSCO.Server.dll --cmd "$@"\n' > /usr/local/bin/msco && chmod +x /usr/local/bin/msco
 VOLUME /data