[.NET MAUI] Focus search bar automatically on search page

[duffh]

Description

It's a better experience to automatically have the search bar focused when navigating to the search page.

Type of change

• Sample viewer enhancement

Platforms tested on

• [ ] WPF .NET 8
• [ ] WPF Framework
• [ ] WinUI
• [x] MAUI WinUI
• [x] MAUI Android
• [x] MAUI iOS
• [x] MAUI MacCatalyst

Checklist

• [x] Self-review of changes
• [x] All changes work as expected on all affected platforms
• [x] There are no warnings related to changes
• [x] Code is commented and follows .NET conventions and standards
• [x] Codemaid and XAML styler extensions have been run on every changed file
• [x] No unrelated changes have been made to any other code or project files