fix: Ensure proper error reporting when a WinAppSDK project references Uno

Author: jeromelaban

build/uno.winui.targets

@@ -10,4 +10,10 @@
 	<Import Project="uno.winui.single-project.targets" />
 	<Import Project="uno.winui.runtime-replace.targets" Condition="'$(WindowsAppSDKWinUI)'!='true'" />
 
+	<Target Name="_WinAppSDKNotSupported"
+			BeforeTargets="BeforeBuild"
+			Condition="'$(WindowsAppSDKWinUI)'=='True'">
+		<Error Code="UNOB0002"
+			   Text="Using Uno.WinUI in a WinAppSDK head project is not supported. You may be including a package that references Uno.WinUI indirectly and it must be removed. See https://aka.platform.uno/UNOB0002 for more details." />
+	</Target>
 </Project>

doc/articles/features/VisibleBoundsPadding.md

@@ -5,6 +5,9 @@ The `ApplicationView.VisibleBounds` is the rectangular area of the screen which
 
 In some cases it's acceptable for visible content to be partially obscured (a page background for example) and it should extend to fill the entire window. Other types of content should be restricted to the visible bounds (for instance readable text, or interactive controls). VisibleBoundsPadding enables this kind of fine-grained control over responsiveness to the visible bounds.
 
+> [!IMPORTANT]
+> The [`SafeArea`](../external/uno.toolkit.ui/doc/controls/SafeArea.md) control is now preferred to `VisibleBoundsPadding`.
+
 ## Using the behavior
 The behavior can be placed on any control that provides a Padding property (e.g. Grid, StackPanel, ListView, ScrollViewer, Control, ContentPresenter or Border), and will be automatically adjusted based on the control's absolute position inside the `ApplicationView.VisibleBounds` rectangle.
 

doc/articles/get-started-wizard.md

@@ -126,3 +126,6 @@ This issue is caused by visual studio enforcing https connections for local cont
 In general, this error happens when the XAML parser detects a syntax error. Fixing the error generally fixes the build.
 
 This error may happen occasionally without any explicit error message, rebuilding the project may fix the issue.
+
+#### Error UNOB0002: Using Uno.WinUI in a WinAppSDK head project is not supported
+This This issue can arise when Uno.WinUI packages are included in a `.Windows` head project, which is not supported. You can fix this by removing the Uno.WinUI or the packages that transitively reference it. For example, including a reference to `Uno.CommunityToolkit.WinUI.*` will transitively include a reference to `Uno.WinUI`, and [the official community toolkit packages](uno-community-toolkit.md) must be used instead.