Start the station link on launch and beacon on every interface

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.Network/StationLink.cs

@@ -1,5 +1,6 @@
 using System.Collections.Concurrent;
 using System.Net;
+using System.Net.NetworkInformation;
 using System.Net.Sockets;
 using System.Text;
 using Nonemm.Core;
@@ -525,9 +526,12 @@ public sealed class StationLink : IDisposable
                 StationBeacon beacon = new(computerName, Address(), port, version, mine.Operator);
                 myAddress = beacon.Address;
                 byte[] message = Encoding.UTF8.GetBytes(beacon.ToWire());
-                await beacons!
-                    .SendAsync(message, new IPEndPoint(IPAddress.Broadcast, port), cancellation)
-                    .ConfigureAwait(false);
+                foreach (IPAddress target in BroadcastAddresses())
+                {
+                    await beacons!
+                        .SendAsync(message, new IPEndPoint(target, port), cancellation)
+                        .ConfigureAwait(false);
+                }
                 mine.Address = beacon.Address;
                 mine.LastHeardUtc = DateTime.UtcNow;
                 Forget();
@@ -1138,6 +1142,38 @@ public sealed class StationLink : IDisposable
         }
     }
 
+    /// The broadcast address of each network this computer is on. The limited
+    /// broadcast (255.255.255.255) leaves by one interface only, so a computer
+    /// with a second adapter or a VPN would miss the stations on the others.
+    private static List<IPAddress> BroadcastAddresses()
+    {
+        List<IPAddress> found = [IPAddress.Broadcast];
+        foreach (NetworkInterface adapter in NetworkInterface.GetAllNetworkInterfaces())
+        {
+            if (adapter.OperationalStatus != OperationalStatus.Up
+                || adapter.NetworkInterfaceType == NetworkInterfaceType.Loopback
+                || !adapter.SupportsMulticast)
+            {
+                continue;
+            }
+            foreach (UnicastIPAddressInformation info in adapter.GetIPProperties().UnicastAddresses)
+            {
+                if (info.Address.AddressFamily != AddressFamily.InterNetwork || info.IPv4Mask is null)
+                {
+                    continue;
+                }
+                byte[] address = info.Address.GetAddressBytes();
+                byte[] mask = info.IPv4Mask.GetAddressBytes();
+                IPAddress directed = new(address.Select((b, i) => (byte)(b | ~mask[i])).ToArray());
+                if (!found.Contains(directed))
+                {
+                    found.Add(directed);
+                }
+            }
+        }
+        return found;
+    }
+
     /// This computer's address on the network it can reach the others on. It is
     /// only for the beacon: the other end uses the address the packet came
     /// from, so a wrong answer here costs nothing.