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.

.github/workflows/host.yml

@@ -0,0 +1,30 @@
+name: Host GUI (Avalonia)
+
+on:
+  push:
+    paths:
+      - "host/**"
+      - ".github/workflows/host.yml"
+  pull_request:
+    paths:
+      - "host/**"
+      - ".github/workflows/host.yml"
+  workflow_dispatch:
+
+jobs:
+  build:
+    name: Build Avalonia host
+    runs-on: [self-hosted, Linux, X64]
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-dotnet@v4
+        # The runner user cannot write to system /usr/share/dotnet; install the
+        # pinned SDK into a runner-writable, cached path instead.
+        env:
+          DOTNET_INSTALL_DIR: ${{ runner.tool_cache }}/dotnet
+        with:
+          dotnet-version: "8.0.x"
+
+      - name: Build host
+        run: dotnet build host/CommonwealthOnline.Host.csproj -c Release --nologo