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>

MSCO.Server.csproj

@@ -7,7 +7,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <RootNamespace>MSCO.Server</RootNamespace>
     <AssemblyName>MSCO.Server</AssemblyName>
-    <Version>2.8.0</Version>
+    <Version>2.9.0</Version>
     <Company>MBM</Company>
     <Product>MSCO Dedicated Server</Product>
     <InvariantGlobalization>true</InvariantGlobalization>