[Settings] Implement singleton pattern for ShortcutConflictWindow (#42440)

## Summary
Fixes issue where multiple ShortcutConflictWindow instances could be
opened simultaneously. The window now follows the same singleton pattern
as OobeWindow - only one instance can exist at a time, and attempting to
open another brings the existing window to the foreground.

## Changes
Implemented singleton management for `ShortcutConflictWindow` following
the established pattern used by `OobeWindow`:

### App.xaml.cs
- Added static field to store the singleton window instance
- Added `GetShortcutConflictWindow()`, `SetShortcutConflictWindow()`,
and `ClearShortcutConflictWindow()` methods for lifecycle management

### ShortcutConflictWindow.xaml.cs
- Updated `WindowEx_Closed` event handler to call
`App.ClearShortcutConflictWindow()` to properly clean up the singleton
reference when the window is closed

### Updated all three entry points that create ShortcutConflictWindow:
- **ShortcutConflictControl.xaml.cs** (Dashboard conflict warning)
- **ShortcutControl.xaml.cs** (Settings page shortcut controls)
- **OobeOverview.xaml.cs** (OOBE overview page)

Each location now checks if a window already exists using
`App.GetShortcutConflictWindow()`:
- If no window exists, creates a new one and registers it via
`App.SetShortcutConflictWindow()`
- If a window already exists, simply calls `Activate()` to bring it to
the foreground

## Testing
The fix ensures that:
- ✅ Only one ShortcutConflictWindow can be open at a time
- ✅ Clicking the shortcut conflict button when a window is already open
activates the existing window instead of creating a duplicate
- ✅ The window reference is properly cleared when closed, allowing a new
instance to be created in future interactions

Fixes #[issue_number]

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[Settings] Single Shortcuts Conflicts
window</issue_title>
> <issue_description>### Microsoft PowerToys version
> 
> 0.95.0
> 
> ### Installation method
> 
> GitHub
> 
> ### Area(s) with issue?
> 
> Settings
> 
> ### Steps to reproduce
> 
> Multiple shortcut conflicts window can be launched.
> Should have the same behavior of OOBE window.
> If shortcut conflicts window is already opened, pressing the button
should bring the window in foreground.
> 
> ### ✔️ Expected Behavior
> 
> Single shortcuts conflicts window
> 
> ### ❌ Actual Behavior
> 
> Multiple shortcut conflicts window can be launched
> 
> ### Additional Information
> 
> _No response_
> 
> ### Other Software
> 
> _No response_</issue_description>
> 
> <agent_instructions>Settings ShortcutConflictWindow should have the
same behavior of OobeWindow.
> When ShortcutConflictWindow is already opened, activate that window
instead of opening another one. </agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

Fixes microsoft/PowerToys#42437

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: davidegiacometti <25966642+davidegiacometti@users.noreply.github.com>
Co-authored-by: Davide Giacometti <davide.giacometti@outlook.it>
Co-authored-by: Niels Laute <niels.laute@live.nl>
Co-authored-by: vanzue <vanzue@outlook.com>

src/settings-ui/Settings.UI/SettingsXAML/Controls/Dashboard/ShortcutConflictControl.xaml.cs

@@ -2,10 +2,7 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
-using System.Collections.Generic;
 using System.ComponentModel;
-using System.Linq;
 using Microsoft.PowerToys.Settings.UI.Library.HotkeyConflicts;
 using Microsoft.PowerToys.Settings.UI.Library.Telemetry.Events;
 using Microsoft.PowerToys.Settings.UI.SettingsXAML.Controls.Dashboard;
@@ -154,11 +151,7 @@ namespace Microsoft.PowerToys.Settings.UI.Controls
                 ConflictCount = this.ConflictCount,
             });
 
-            // Create and show the new window instead of dialog
-            var conflictWindow = new ShortcutConflictWindow();
-
-            // Show the window
-            conflictWindow.Activate();
+            ((App)App.Current)!.OpenShortcutConflictWindow();
         }
     }
 }

src/settings-ui/Settings.UI/SettingsXAML/Controls/Dashboard/ShortcutConflictWindow.xaml.cs

@@ -2,16 +2,13 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
 using CommunityToolkit.WinUI.Controls;
 using Microsoft.PowerToys.Settings.UI.Helpers;
 using Microsoft.PowerToys.Settings.UI.Library;
-using Microsoft.PowerToys.Settings.UI.Library.Helpers;
 using Microsoft.PowerToys.Settings.UI.Library.HotkeyConflicts;
 using Microsoft.PowerToys.Settings.UI.Services;
 using Microsoft.PowerToys.Settings.UI.ViewModels;
 using Microsoft.PowerToys.Settings.UI.Views;
-using Microsoft.UI;
 using Microsoft.UI.Windowing;
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
@@ -26,7 +23,11 @@ namespace Microsoft.PowerToys.Settings.UI.SettingsXAML.Controls.Dashboard
 
         public ShortcutConflictWindow()
         {
+            App.ThemeService.ThemeChanged += OnThemeChanged;
+            App.ThemeService.ApplyTheme();
+
             var settingsUtils = SettingsUtils.Default;
+
             ViewModel = new ShortcutConflictViewModel(
                 settingsUtils,
                 SettingsRepository<GeneralSettings>.GetInstance(settingsUtils),
@@ -50,6 +51,11 @@ namespace Microsoft.PowerToys.Settings.UI.SettingsXAML.Controls.Dashboard
             ViewModel.OnPageLoaded();
         }
 
+        private void OnThemeChanged(object sender, ElementTheme theme)
+        {
+            WindowHelper.SetTheme(this, theme);
+        }
+
         private void CenterOnScreen()
         {
             var displayArea = DisplayArea.GetFromWindowId(this.AppWindow.Id, DisplayAreaFallback.Nearest);
@@ -127,6 +133,14 @@ namespace Microsoft.PowerToys.Settings.UI.SettingsXAML.Controls.Dashboard
         private void WindowEx_Closed(object sender, WindowEventArgs args)
         {
             ViewModel?.Dispose();
+
+            var mainWindow = App.GetSettingsWindow();
+            if (mainWindow != null)
+            {
+                mainWindow.CloseHiddenWindow();
+            }
+
+            App.ThemeService.ThemeChanged -= OnThemeChanged;
         }
 
         private void Window_Activated_SetIcon(object sender, WindowActivatedEventArgs args)

src/settings-ui/Settings.UI/SettingsXAML/Controls/ShortcutControl/ShortcutControl.xaml.cs

@@ -9,7 +9,6 @@ using System.Linq;
 using CommunityToolkit.WinUI;
 using Microsoft.PowerToys.Settings.UI.Helpers;
 using Microsoft.PowerToys.Settings.UI.Library;
-using Microsoft.PowerToys.Settings.UI.Library.HotkeyConflicts;
 using Microsoft.PowerToys.Settings.UI.Library.Telemetry.Events;
 using Microsoft.PowerToys.Settings.UI.Services;
 using Microsoft.PowerToys.Settings.UI.SettingsXAML.Controls.Dashboard;
@@ -300,9 +299,7 @@ namespace Microsoft.PowerToys.Settings.UI.Controls
             // Close the current shortcut dialog
             shortcutDialog.Hide();
 
-            // Create and show the ShortcutConflictWindow
-            var conflictWindow = new ShortcutConflictWindow();
-            conflictWindow.Activate();
+            ((App)App.Current)!.OpenShortcutConflictWindow();
         }
 
         private void UpdateKeyVisualStyles()