Shadcn Guideline: Toast — Add Toaster to layout. Description: Include Toaster component in root layout. Do: <Toaster /> in app layout. Don't: Toaster in individual pages. Good Example: app/layout.tsx: <Toaster />. Bad Example: page.tsx: <Toaster />. Severity: High. Docs: https://ui.shadcn.com/docs/components/sonner.