WinUI3 dotnet new tempalte updates

[nmetulev]

Quality-of-life improvements to the WinUI 3 dotnet new template pack to align with modern .NET conventions and improve the new project experience for users and AI agents.

Changes

1. Frame + MainPage architecture (blank app template)

• MainWindow now hosts a instead of an empty , with MainPage as the default content
• Prevents a common pitfall where users/agents try to use Page-only features directly on Window
• Added guiding comments in XAML and code-behind directing users to add their UI in MainPage

2. Pinned NuGet package versions with auto-update

• Replaced Version="*" with pinned stable versions (Microsoft.WindowsAppSDK
  1.8.260317003, Microsoft.Windows.SDK.BuildTools 10.0.26100.7705) as an offline fallback
• Added addPackageReference post-actions that run dotnet add package at scaffold time to resolve the latest stable version from NuGet automatically
• Both winui and winui-navview templates benefit; continueOnError: true ensures offline scaffolding still works
• Also fixed the NavigationApp csproj which had an unconditional workaround block — now uses the proper #if conditional

3. .gitignore included in all project templates

• All four project templates (winui, winui-navview, winui-lib, winui-unittest) now scaffold a .gitignore covering .NET, Visual Studio, MSIX, and common tooling artifacts
• Single shared source file (templates/gitignore.txt) mapped via template engine rename rules

4. Implicit usings enabled

• Added enable to all four project template csproj files
• Removed redundant System.* usings from all .cs files (now auto-imported by the SDK)
• Kept Microsoft.UI.Xaml.* usings in App/Window code-behind files for discoverability

5. winui3 short name aliases

• All templates now accept winui3-* as an alias (e.g., dotnet new winui3, dotnet new winui3-navview)
• Uses shortName as a JSON array — the shortNameList field was not honored by the CLI host

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