fix: add missing System.Collections.Frozen using in App.xaml.cs

PR openclaw#105 introduced ToFrozenDictionary but missed the using directive
for the WinUI project. Fixes build break on master.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: shanselman

src/OpenClaw.Tray.WinUI/App.xaml.cs

@@ -8,6 +8,7 @@
 using OpenClawTray.Services;
 using OpenClawTray.Windows;
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Drawing;