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/CheckWindow.axaml.cs

@@ -0,0 +1,76 @@
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Media;
+using Nonemm.Session;
+
+namespace Nonemm.App.Windows;
+
+/// What the callsign being typed could be, a column per source. The columns
+/// stay apart because they answer different questions, and merged into one list
+/// they would all read as equally reliable.
+public sealed partial class CheckWindow : RefreshableWindow
+{
+    private readonly AppSession session;
+    private readonly Func<string> typed;
+
+    public CheckWindow(AppSession session, Func<string> typed)
+    {
+        this.session = session;
+        this.typed = typed;
+        InitializeComponent();
+        Refresh();
+    }
+
+
+    public override void Refresh()
+    {
+        Columns.Children.Clear();
+        if (session.Check is null)
+        {
+            return;
+        }
+        int at = 0;
+        foreach (CheckColumn column in session.Check.Columns(typed()))
+        {
+            Control panel = BuildColumn(column);
+            Grid.SetColumn(panel, at++);
+            Columns.Children.Add(panel);
+        }
+    }
+
+    private static Control BuildColumn(CheckColumn column)
+    {
+        StackPanel panel = new() { Margin = new Avalonia.Thickness(0, 0, 8, 0) };
+        panel.Children.Add(new TextBlock
+        {
+            Text = $"{Heading(column.Source)} {Count(column)}",
+            FontSize = 11,
+            Opacity = 0.7,
+            Margin = new Avalonia.Thickness(0, 0, 0, 4),
+        });
+        foreach (CheckCandidate candidate in column.Candidates)
+        {
+            panel.Children.Add(new TextBlock
+            {
+                Text = candidate.Call,
+                FontFamily = new FontFamily("monospace"),
+                FontSize = 14,
+                Foreground = Verdicts.Colour(candidate.Verdict),
+                HorizontalAlignment = HorizontalAlignment.Left,
+            });
+        }
+        return new ScrollViewer { Content = panel };
+    }
+
+    /// Before anything is typed the heading says how many calls the source
+    /// holds; once it is offering candidates it says how many.
+    private static string Count(CheckColumn column) =>
+        column.Candidates.Count > 0 ? column.Candidates.Count.ToString() : $"({column.Held})";
+
+    private static string Heading(CheckSource source) => source switch
+    {
+        CheckSource.Log => "Log",
+        CheckSource.Database => "Master",
+        _ => "Bandmap",
+    };
+}