Add Avalonia server-host launcher (MVP scaffold)

Cross-platform C# GUI to replace the Qt/C++ Host GUI, on the same dotnet
toolchain as the server. MVP: load/save commonwealth-server.json, Start/Stop
the CommonwealthOnline.Server process (published exe, then dll, then source
run), and stream its output to a live log. Themed to the Commonwealth Online
brand (near-black + amber). Builds clean on net8.0.

Next: player list + kick/ban via the admin port, then retire the Qt host.

host/App.axaml

@@ -0,0 +1,25 @@
+<Application xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             x:Class="CommonwealthOnline.Host.App"
+             RequestedThemeVariant="Dark">
+
+  <Application.Styles>
+    <FluentTheme />
+  </Application.Styles>
+
+  <Application.Resources>
+    <ResourceDictionary>
+      <Color x:Key="CoBackground">#0E120B</Color>
+      <Color x:Key="CoSurface">#171C10</Color>
+      <Color x:Key="CoAccent">#E6D28C</Color>
+      <Color x:Key="CoText">#D8D2BE</Color>
+      <Color x:Key="CoDanger">#C24B4B</Color>
+      <SolidColorBrush x:Key="CoBackgroundBrush" Color="{StaticResource CoBackground}" />
+      <SolidColorBrush x:Key="CoSurfaceBrush" Color="{StaticResource CoSurface}" />
+      <SolidColorBrush x:Key="CoAccentBrush" Color="{StaticResource CoAccent}" />
+      <SolidColorBrush x:Key="CoTextBrush" Color="{StaticResource CoText}" />
+      <SolidColorBrush x:Key="CoDangerBrush" Color="{StaticResource CoDanger}" />
+    </ResourceDictionary>
+  </Application.Resources>
+
+</Application>