use Windows NetworkConnect stock icon for form and tray

SystemIcons.GetStockIcon(StockIconId.NetworkConnect, 32) — no
external files needed.

win/gatuna-client/MainForm.cs

@@ -18,6 +18,7 @@ public partial class MainForm : Form
         Width = 520;
         Height = 380;
         StartPosition = FormStartPosition.CenterScreen;
+        Icon = SystemIcons.GetStockIcon(StockIconId.NetworkConnect, 32);
         InitializeComponents();
 
         _sessions.Log += msg => this.Invoke(() => _statusLabel.Text = msg);

win/gatuna-client/Program.cs

@@ -11,7 +11,7 @@ static class Program
 
         using var tray = new NotifyIcon
         {
-            Icon = SystemIcons.Application,
+            Icon = SystemIcons.GetStockIcon(StockIconId.NetworkConnect, 32),
             Text = "gatuna",
             Visible = true,
         };