Fix Settings crash: eliminate off-UI-thread DependencyProperty updates in conflict detection

The crash (STATUS_STOWED_EXCEPTION / 0xc000027b in CoreMessagingXP.dll) was caused by
two threading bugs in the hotkey conflict detection feature:

1. PageViewModelBase.OnConflictsUpdated: The original code had a Task.Run with a
   catch block that called UpdateConflictProperties() directly on a Task thread when
   settingsWindow was null. UpdateConflictProperties() sets HotkeySettings.HasConflict
   and HotkeySettings.ConflictDescription, which raise PropertyChanged. With active
   x:Bind bindings, WinUI3 propagated those notifications as a stowed exception through
   CoreMessagingXP.dll, crashing the process.

   Fixed by removing the unsafe catch-block fallback and the unnecessary Task.Run
   wrapper. TryEnqueue is non-blocking and thread-safe, so it can be called directly
   on the IPC thread. If the settings window is null, the update is skipped (correct
   behavior when there is no UI to update).

2. ShortcutConflictViewModel: Used WPF's System.Windows.Threading.Dispatcher
   (Dispatcher.CurrentDispatcher + BeginInvoke) in a WinUI3 app, mixing threading
   models. Replaced with Microsoft.UI.Dispatching.DispatcherQueue captured via
   DispatcherQueue.GetForCurrentThread() in the constructor, matching the pattern
   used by DashboardViewModel.

Agent-Logs-Url: https://github.com/microsoft/PowerToys/sessions/2dc7795f-f2bf-4e8d-992b-2a5deeefae23

Co-authored-by: MuyuanMS <116717757+MuyuanMS@users.noreply.github.com>

src/settings-ui/Settings.UI/ViewModels/PageViewModelBase.cs

@@ -6,7 +6,6 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using System.Threading.Tasks;
 using Microsoft.PowerToys.Settings.UI.Helpers;
 using Microsoft.PowerToys.Settings.UI.Library;
 using Microsoft.PowerToys.Settings.UI.Library.Helpers;
@@ -71,18 +70,8 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
                 }
             }
 
-            _ = Task.Run(() =>
-            {
-                try
-                {
-                    var settingsWindow = App.GetSettingsWindow();
-                    settingsWindow.DispatcherQueue.TryEnqueue(Microsoft.UI.Dispatching.DispatcherQueuePriority.Normal, UpdateConflictProperties);
-                }
-                catch
-                {
-                    UpdateConflictProperties();
-                }
-            });
+            var dispatcherQueue = App.GetSettingsWindow()?.DispatcherQueue;
+            dispatcherQueue?.TryEnqueue(Microsoft.UI.Dispatching.DispatcherQueuePriority.Normal, UpdateConflictProperties);
         }
 
         public virtual Dictionary<string, HotkeySettings[]> GetAllHotkeySettings()

src/settings-ui/Settings.UI/ViewModels/ShortcutConflictViewModel.cs

@@ -12,8 +12,8 @@ using System.Reflection;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Text.Json.Serialization.Metadata;
-using System.Windows.Threading;
 using ManagedCommon;
+using Microsoft.UI.Dispatching;
 using Microsoft.PowerToys.Settings.UI.Helpers;
 using Microsoft.PowerToys.Settings.UI.Library;
 using Microsoft.PowerToys.Settings.UI.Library.Helpers;
@@ -29,7 +29,7 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
     {
         private readonly SettingsFactory _settingsFactory;
         private readonly Func<string, int> _ipcMSGCallBackFunc;
-        private readonly Dispatcher _dispatcher;
+        private readonly DispatcherQueue _dispatcherQueue;
 
         private bool _disposed;
         private AllHotkeyConflictsData _conflictsData = new();
@@ -41,7 +41,7 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
             ISettingsRepository<GeneralSettings> settingsRepository,
             Func<string, int> ipcMSGCallBackFunc)
         {
-            _dispatcher = Dispatcher.CurrentDispatcher;
+            _dispatcherQueue = DispatcherQueue.GetForCurrentThread();
             _ipcMSGCallBackFunc = ipcMSGCallBackFunc ?? throw new ArgumentNullException(nameof(ipcMSGCallBackFunc));
             resourceLoader = ResourceLoaderInstance.ResourceLoader;
 
@@ -123,7 +123,7 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
 
         protected override void OnConflictsUpdated(object sender, AllHotkeyConflictsEventArgs e)
         {
-            _dispatcher.BeginInvoke(() =>
+            _dispatcherQueue?.TryEnqueue(() =>
             {
                 ConflictsData = e.Conflicts ?? new AllHotkeyConflictsData();
             });