Svelte Guideline: Component Library — Use shadcn-svelte + bits-ui stack. Description: Three-tier UI: bits-ui primitives > shadcn wrappers > app components. Do: Check shadcn catalog before building custom; components in src/lib/components/ui/. Don't: Building custom components from scratch when shadcn has one. Good Example: import { Button } from '$lib/components/ui/button'. Bad Example: <button class='custom-btn'>. Severity: High. Docs: https://www.shadcn-svelte.com.