Fix: Modal window sample not scaled to dpi (#2147)

<!--- Provide a general summary of your changes in the Title above -->

## Description
The Modal window example size is not scaled to dpi currently. On a high
dpi screen,the window content is not even complete. This PR fixes it,
together with sample code.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here. -->

## How Has This Been Tested?
<!--- Please describe in detail how you tested your changes. -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

## Screenshots (if appropriate):
Before:

<img width="370" height="285" alt="image"
src="https://github.com/user-attachments/assets/46b358a9-cf5a-49ea-887c-ba6e14e98387"
/>

After:

<img width="1170" height="885" alt="image"
src="https://github.com/user-attachments/assets/df01a450-553b-45b0-b831-64905705c202"
/>


## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

Author: HO-COOH

WinUIGallery/Samples/SampleCode/AppWindow/AppWindowSample5_cs.txt

@@ -12,7 +12,11 @@ public sealed partial class ModalWindow : Window
     public ModalWindow()
     {
         this.InitializeComponent();
-        AppWindow.Resize(new Windows.Graphics.SizeInt32(400, 300));
+
+        var dpi = Windows.Win32.PInvoke.GetDpiForWindow(new Windows.Win32.Foundation.HWND(WinRT.Interop.WindowNative.GetWindowHandle(this)));
+        var scalingFactor = (float)dpi / 96;
+
+        AppWindow.Resize(new Windows.Graphics.SizeInt32((int)(400.0f * scalingFactor), (int)(300.0f * scalingFactor)));
         AppWindow.SetIcon("Assets/Tiles/GalleryIcon.ico");
         AppWindow.TitleBar.PreferredTheme = TitleBarTheme.UseDefaultAppMode;
 

WinUIGallery/Samples/SamplePages/ModalWindow.xaml.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
 using Microsoft.UI;
@@ -15,7 +15,11 @@ public sealed partial class ModalWindow : Window
     public ModalWindow()
     {
         this.InitializeComponent();
-        AppWindow.Resize(new Windows.Graphics.SizeInt32(400, 300));
+
+        var dpi = Windows.Win32.PInvoke.GetDpiForWindow(new Windows.Win32.Foundation.HWND(WinRT.Interop.WindowNative.GetWindowHandle(this)));
+        var scalingFactor = (float)dpi / 96;
+
+        AppWindow.Resize(new Windows.Graphics.SizeInt32((int)(400.0f * scalingFactor), (int)(300.0f * scalingFactor)));
         AppWindow.SetIcon("Assets/Tiles/GalleryIcon.ico");
         AppWindow.TitleBar.PreferredTheme = TitleBarTheme.UseDefaultAppMode;