feat(ecommerce): selector de variante inline en tarjeta de producto

Reutiliza Product.variants[].{name,value,available}, ya expuesto por el
provider de Medusa (variant.options[0]), para swatches clicables en la
tarjeta del catalogo/home -- no solo en la ficha de producto. Solo se
muestra cuando el producto tiene mas de una variante; la variante
seleccionada se usa al agregar al carrito. ADR 0005 seccion 3 (patron
eBay), cero cambio de modelo de datos.

workloads/ecommerce/components/product/product-card.tsx

@@ -3,6 +3,7 @@
 import Image from "next/image";
 import Link from "next/link";
 import { Heart, ShoppingBag, Sparkles } from "lucide-react";
+import { useState } from "react";
 import { Badge } from "@/components/ui/badge";
 import { Button } from "@/components/ui/button";
 import type { Product } from "@/lib/headless/types";
@@ -21,6 +22,8 @@ export function ProductCard({ product, priority = false }: { product: Product; p
       ? Math.round((1 - product.price / product.compareAtPrice) * 100)
       : null;
 
+  const [selectedVariantId, setSelectedVariantId] = useState(product.variants[0]?.id);
+
   return (
     <article className="group flex h-full flex-col overflow-hidden rounded-3xl border border-slate-100 bg-white shadow-lg shadow-slate-900/5 transition duration-300 hover:-translate-y-1.5 hover:shadow-2xl hover:shadow-violet-500/10">
       <div className="relative aspect-square overflow-hidden bg-slate-50">
@@ -32,12 +35,33 @@ export function ProductCard({ product, priority = false }: { product: Product; p
         <div className="flex items-center justify-between gap-2 text-xs font-bold text-slate-500"><span>{product.category}</span><span className="rounded-full bg-cyan-50 px-2 py-1 text-cyan-700">{product.collection}</span></div>
         <Link href={`/product/${product.slug}`} prefetch={false}><h3 className="line-clamp-2 min-h-12 text-lg font-black leading-6 text-slate-950 group-hover:text-fuchsia-600">{product.name}</h3></Link>
         <p className="line-clamp-2 min-h-10 text-sm leading-5 text-slate-500">{product.shortDescription}</p>
+        {product.variants.length > 1 && (
+          <div className="flex flex-wrap gap-1.5">
+            {product.variants.map((variant) => (
+              <button
+                key={variant.id}
+                type="button"
+                disabled={!variant.available}
+                aria-pressed={selectedVariantId === variant.id}
+                aria-label={`${variant.name}: ${variant.value}`}
+                onClick={() => setSelectedVariantId(variant.id)}
+                className={`rounded-full border px-2.5 py-1 text-xs font-bold transition ${
+                  selectedVariantId === variant.id
+                    ? "border-fuchsia-500 bg-fuchsia-50 text-fuchsia-700"
+                    : "border-slate-200 text-slate-600 hover:border-slate-400"
+                } ${!variant.available ? "cursor-not-allowed opacity-40" : ""}`}
+              >
+                {variant.value}
+              </button>
+            ))}
+          </div>
+        )}
         <div className="mt-auto flex items-center justify-between gap-3 pt-1">
           <div className="flex flex-col">
             {product.compareAtPrice && <span className="text-xs font-bold text-slate-400 line-through">{formatMoney(product.compareAtPrice, product.currency)}</span>}
             <strong className="text-sm font-black">{formatMoney(product.price, product.currency)}</strong>
           </div>
-          <Button variant="magenta" size="sm" onClick={() => addItem(product, product.variants[0]?.id)}><ShoppingBag className="size-4" />Agregar</Button>
+          <Button variant="magenta" size="sm" onClick={() => addItem(product, selectedVariantId)}><ShoppingBag className="size-4" />Agregar</Button>
         </div>
       </div>
     </article>