Add the Avalonia application and the station network

The entry window types contacts and colours them as they are typed; the log,
check, bandmap, score and packet windows read the same session. Contacts are
shared with the other stations of a multi-operator entry in N1MM's contact
message, so an N1MM station on the same network sees them too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/Nonemm.App/Windows/RefreshableWindow.cs

@@ -0,0 +1,9 @@
+using Avalonia.Controls;
+
+namespace Nonemm.App.Windows;
+
+/// A window the entry window tells to redraw when the log changes.
+public abstract class RefreshableWindow : Window
+{
+    public abstract void Refresh();
+}