SonnerService.Clear() and a destructive ContextMenuItem #61

Summary

Two small API gaps:

1. SonnerService has no way to dismiss every toast. Add Clear() (dismiss all, with the exit animation).
2. ContextMenuItem has no destructive style for items like "Delete". Add Destructive (red text, like shadcn's variant="destructive").

Both in the package and the CLI templates, with tests.