ApplicationIcon setting doesn't work

### Describe the bug

When I set the icon for my WinUI app in the project file, it doesn't work.
```cs
Assets\AppIcon.ico
```

### Why is this important?

This is a basic window ui design requirement. I'm actually shocked that a 14 year old UI framework doesn't provide a way to set the application icon. That just seems insane to me, there is something really wrong at MS when you are missing such basic features in your tools.

### Steps to reproduce the bug

1. Add an icon file to Assets folder, eg. AppIcon.ico
2. Configure icon:
```xml
Assets\AppIcon.ico
```
3. Run app and observe that no icons have changed, either in the taskbar or titlebar.
4. Check the release date of WinUI 3.
5. Get frustrated that bing keeps telling you about the WinUI 3 open sourcing efforts.
6. Check WinUI wiki, and see that the framework is 14 years old.
7. Shake your head in disbelief and wonder to yourself 'this just couldn't be the case - they've never added the ability to set the application icon!?'
8. Get a little bit angry.

### NuGet package version

WinUI 3 - Windows App SDK 1.8.2: 1.8.251003001

### Windows version

Windows 11 (24H2): Build 26100

### Additional Context

If you haven't added the ability to change the application icon in 14 years, you need to double the team size on WinUI immediately and possibly restructure your cross-team communication, because you've got serious problems.

No source file, test, or entry point is named. Start by locating how the WinUI 3/Windows App SDK project handles the ApplicationIcon property and reproduce the behavior with Assets\AppIcon.ico; done means the configured icon appears in the taskbar and title bar.

Written by the indexing model from the issue text.