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/Configuration/UserPaths.cs

@@ -0,0 +1,47 @@
+namespace Nonemm.App.Configuration;
+
+/// Where the operator's files live. Windows keeps them under Documents the way
+/// N1MM does; everywhere else follows the XDG configuration directory.
+public sealed class UserPaths
+{
+    public UserPaths(string root)
+    {
+        Root = root;
+        Databases = Path.Combine(root, "Databases");
+        UserDefinedContests = Path.Combine(root, "UserDefinedContests");
+        SupportFiles = Path.Combine(root, "SupportFiles");
+    }
+
+    public static UserPaths Default() => new(DefaultRoot());
+
+    public string Root { get; }
+
+    public string Databases { get; }
+
+    public string UserDefinedContests { get; }
+
+    public string SupportFiles { get; }
+
+    public string SettingsFile => Path.Combine(Root, "settings.json");
+
+    public string CountryFile => Path.Combine(SupportFiles, "wl_cty.dat");
+
+    public string CallDatabaseFile => Path.Combine(SupportFiles, "MASTER.SCP");
+
+    public void CreateFolders()
+    {
+        Directory.CreateDirectory(Root);
+        Directory.CreateDirectory(Databases);
+        Directory.CreateDirectory(UserDefinedContests);
+        Directory.CreateDirectory(SupportFiles);
+    }
+
+    private static string DefaultRoot() =>
+        OperatingSystem.IsWindows()
+            ? Path.Combine(
+                Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments),
+                "Nonemm")
+            : Path.Combine(
+                Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+                "nonemm");
+}