CLI: support NuGet dependencies so components can use ShellIcons #30

Summary

Some components need a NuGet package: qr-code needs a QR library, and icons should come from ShellIcons.Maui / ShellIcons.Avalonia instead of the built-in Icon.cs set (which is missing common icons such as Receipt, ListChecks and CalendarClock). The CLI can't add package references yet.

Proposal

Let a template declare NuGet dependencies (id and version), and have add and update add the PackageReference when it's missing (without downgrading an existing higher version). Then:

1. move icon to ShellIcons (keeping the current Kind names working or mapping them), and
2. add qr-code.