Add WebSocket heartbeat, client cleanup, and connection toast notifications

- Add heartbeat ping every 5 seconds to detect stale connections
- Replace clients when same IP reconnects instead of accumulating
- Remove unavailable clients on connect and heartbeat probe
- Marshal UI updates to UI thread via SynchronizationContext
- Show balloon toast on device connect/disconnect
- Add disposal safety check for UpdateTrayStatus

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

WirelessTextSyncer.Windows/Tray/TrayApplicationContext.cs

@@ -10,17 +10,21 @@ public sealed class TrayApplicationContext : ApplicationContext
     private readonly NotifyIcon notifyIcon;
     private readonly Icon connectedIcon;
     private readonly Icon waitIcon;
+    private readonly SynchronizationContext uiContext;
     private ToolStripMenuItem? clipboardPasteMenuItem;
     private ToolStripMenuItem? sendInputTypingMenuItem;
+    private bool? lastConnectionState;
+    private bool disposed;
 
     public TrayApplicationContext()
     {
         AppLogger.Info("Tray application starting.");
+        uiContext = SynchronizationContext.Current ?? new WindowsFormsSynchronizationContext();
         keyboard = new KeyboardInjectionService();
         var handler = new SyncMessageHandler(keyboard);
 
         server = new WebSocketServerService(handler);
-        server.StatusChanged += (_, _) => UpdateTrayStatus();
+        server.StatusChanged += (_, _) => uiContext.Post(_ => UpdateTrayStatus(), null);
         discovery = new DiscoveryResponderService(() => server.LocalIpAddress, () => server.Port);
         connectedIcon = TrayIconFactory.CreateConnectedIcon();
         waitIcon = TrayIconFactory.CreateWaitIcon();
@@ -53,6 +57,7 @@ public sealed class TrayApplicationContext : ApplicationContext
     {
         if (disposing)
         {
+            disposed = true;
             notifyIcon.Dispose();
             connectedIcon.Dispose();
             waitIcon.Dispose();
@@ -139,10 +144,32 @@ public sealed class TrayApplicationContext : ApplicationContext
 
     private void UpdateTrayStatus()
     {
+        if (disposed)
+        {
+            return;
+        }
+
         var connected = server.HasClient;
         var status = connected ? "connected" : "waiting";
         var text = $"WirelessTextSyncer {server.LocalIpAddress}:{server.Port} ({status})";
         notifyIcon.Icon = connected ? connectedIcon : waitIcon;
         notifyIcon.Text = text.Length > 63 ? text[..63] : text;
+
+        if (lastConnectionState is not null && lastConnectionState != connected)
+        {
+            ShowConnectionToast(connected);
+        }
+
+        lastConnectionState = connected;
+    }
+
+    private void ShowConnectionToast(bool connected)
+    {
+        notifyIcon.BalloonTipTitle = "WirelessTextSyncer";
+        notifyIcon.BalloonTipText = connected
+            ? $"Device connected to {server.LocalIpAddress}:{server.Port}."
+            : "Device disconnected. Waiting for connection.";
+        notifyIcon.BalloonTipIcon = connected ? ToolTipIcon.Info : ToolTipIcon.Warning;
+        notifyIcon.ShowBalloonTip(3000);
     }
 }