ShellLabel: add HorizontalTextAlignment and LineBreakMode #27

Summary

ShellLabel has no HorizontalTextAlignment or LineBreakMode, so centred or wrapping text (help text under a form, empty states) needs a plain MAUI Label with {DynamicResource ShellUIMutedForeground} instead.

Task

Add pass-through bindable properties for HorizontalTextAlignment and LineBreakMode (MAUI) and the equivalents (TextAlignment, TextWrapping) on Avalonia, with tests.