Migrate to the cn package

- Ran `npx shadcn@latest migrate cn`: swaps clsx + tailwind-merge for cn
- Import cn from "cn" everywhere (matches what `shadcn add` now writes)
  and remove the src/lib/utils.js re-export shim

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/components/ui/alert.jsx

@@ -1,6 +1,6 @@
 import * as React from "react"
 import { cva } from "class-variance-authority";
-import { cn } from "@/lib/utils"
+import { cn } from "cn"
 
 const alertVariants = cva(
   "relative grid w-full grid-cols-[0_1fr] items-start gap-y-0.5 rounded-lg border px-4 py-3 text-sm has-[>svg]:grid-cols-[calc(var(--spacing)*4)_1fr] has-[>svg]:gap-x-3 [&>svg]:size-4 [&>svg]:translate-y-0.5 [&>svg]:text-current",