fix: copy Assets folder in RID-specific and publish builds

[pexatar]

without CopyToOutputDirectory is handled by WinUI 3 targets only for the default (non-RID) output. Self-contained publish with -r win-x64 and RID-specific builds silently skip Assets, causing DirectoryNotFoundException at runtime for PassKey.ico.

Added CopyToOutputDirectory and CopyToPublishDirectory=PreserveNewest so Assets (PassKey.ico, logos) are deployed in all build configurations.

Description

Brief description of the changes in this PR.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Testing

Describe the tests you ran to verify your changes.

• All existing tests pass (dotnet test)
• New tests added for new functionality
• Solution builds without warnings (dotnet build PassKey.sln -p:Platform=x64)

Checklist

• My code follows the project's code style guidelines
• I have added XML documentation to all new public types and members
• I have updated the CHANGELOG.md if applicable
• My changes do not introduce new compiler warnings