Retire the Qt/C++ host; ship the Avalonia host

Delete the Qt host (src/, cmake/, CMakeLists.txt, find-qt.bat, check-setup.bat,
build.bat, deploy.bat) now that the Avalonia host in host/ covers it on the same
dotnet toolchain. Add a self-hosted host-build CI job and a host README, and
rewrite the top-level README for the C# server + Avalonia host layout.

host/README.md

@@ -0,0 +1,31 @@
+# Commonwealth Online — Server Host (Avalonia)
+
+Cross-platform C# GUI (.NET 8 + Avalonia) for running a Commonwealth Online
+dedicated server. Replaces the former Qt/C++ host, so the whole project now
+builds on one `dotnet` toolchain.
+
+## Run
+
+```
+dotnet run --project host/CommonwealthOnline.Host.csproj
+```
+
+Launch it from (or point its working directory at) a folder that holds the
+server — a published `CommonwealthOnline.Server` executable, the framework
+`CommonwealthOnline.Server.dll`, or a source checkout — alongside
+`commonwealth-server.json`.
+
+## Features
+
+- Edit and save `commonwealth-server.json` (host, port, name, max players,
+  admin port, log verbosity, GNS toggle).
+- Start / Stop the server and stream its output to a live log.
+- Live player list with **Kick** / **Ban**, driven by the server's
+  token-authenticated admin port.
+
+## Publish
+
+```
+dotnet publish host/CommonwealthOnline.Host.csproj -c Release -r win-x64 --self-contained false
+dotnet publish host/CommonwealthOnline.Host.csproj -c Release -r linux-x64 --self-contained false
+```