Windows: Shell.Background behavior is inconsistent with Android and iOS for TabBar

### Description

Shell.Background and Shell.BackgroundColor behave inconsistently across platforms.
On Android and iOS, the value assigned to Shell.Background is applied consistently to both:

- The navigation bar (toolbar)
- The bottom tab bar

However, on Windows, the same property is applied only to the navigation bar, while the tab bar continues to use its default background color.

Note: This behavior is observed only when no explicit styles are defined

**Actual Behavior**

- Android, iOS: The background is applied to both the navigation bar and the bottom tab bar.
- Windows: The background is applied only to the navigation bar. The tab bar retains its default appearance.

**Expected Behavior**
The behavior should be consistent across platforms. Either:

- Shell.Background should affect both the navigation bar and the tab bar on all platforms, or
- the documented behavior should clearly specify any platform-specific differences.

### Steps to Reproduce

1. Create a .NET MAUI application that uses Shell.
2. Set Shell.Background or Shell.BackgroundColor in AppShell.xaml.
3. Run the application on Android, iOS, and Windows.
4. Compare the appearance of the navigation bar and tab bar.

[ShellGradient.zip](https://github.com/user-attachments/files/30783275/ShellGradient.zip)

### Link to public reproduction project repository

_No response_

### Version with bug

11.0.0-preview.6

### Is this a regression from previous behavior?

Not sure, did not test other versions

### Last version that worked well

_No response_

### Affected platforms

Windows

### Affected platform versions

_No response_

### Did you find any workaround?

_No response_

### Relevant log output

```shell

```

Start with the AppShell.xaml configuration described in the issue and run the attached ShellGradient.zip reproduction on Android, iOS, and Windows. Compare Shell.Background and Shell.BackgroundColor on the navigation bar and bottom tab bar, then verify that the behavior is consistent across platforms or that the platform-specific difference is documented.

Written by the indexing model from the issue text.