Keep the theme in sync between native and Blazor UI in hybrid apps #29

Summary

In a MAUI Blazor Hybrid app, ShellTheme (native) and the Blazor dark class on <html> change independently, so toggling one leaves the other on the wrong theme.

Proposal

A small helper, or a documented pattern:

• Blazor → native: a JS → .NET callback from the Blazor ThemeToggle that sets ShellTheme.
• Native → Blazor: when ShellTheme changes, run a script in the BlazorWebView that sets or removes the dark class.

Also document how to mirror a custom Blazor theme (tweakcn or hex values in input.css) into ShellTheme overrides, so both halves use the same colours.