Add SEO improvements: sitemap, robots.txt, LocalBusiness schema, GA4, llm.txt

- Add dynamic sitemap.xml generation for all pages
- Add robots.txt for search engine crawl directives
- Add LocalBusiness JSON-LD schema for local SEO
- Add BreadcrumbList schema for navigation breadcrumbs
- Add canonical URLs to all product pages
- Add Twitter Cards metadata
- Add Google Analytics 4 integration component
- Create llm.txt with all product data for AI optimization
- Create reusable UI components (Button, Card, Badge)
- Update company address to full Thai address
- Update .env.example with GA4 placeholder

src/app/[...slug]/page.tsx

@@ -69,11 +69,15 @@ export async function generateMetadata({ params }: Props) {
       title,
       description: product.description,
       keywords: product.keywords?.join(', '),
+      alternates: {
+        canonical: product.href,
+      },
       openGraph: {
         title: product.name,
         description: product.description,
         images: [product.image],
         type: 'website',
+        url: `https://dealplustech.co.th${product.href}`,
       },
     };
   }
@@ -123,6 +127,41 @@ function ProductSchema({ product }: { product: ProductCategory }) {
   );
 }
 
+// BreadcrumbList Schema for SEO
+function BreadcrumbSchema({ product }: { product: ProductCategory }) {
+  const schema = {
+    '@context': 'https://schema.org',
+    '@type': 'BreadcrumbList',
+    itemListElement: [
+      {
+        '@type': 'ListItem',
+        position: 1,
+        name: 'หน้าแรก',
+        item: 'https://dealplustech.co.th',
+      },
+      {
+        '@type': 'ListItem',
+        position: 2,
+        name: 'สินค้า',
+        item: 'https://dealplustech.co.th/product/',
+      },
+      {
+        '@type': 'ListItem',
+        position: 3,
+        name: product.name,
+        item: `https://dealplustech.co.th${product.href}`,
+      },
+    ],
+  };
+
+  return (
+    <script
+      type="application/ld+json"
+      dangerouslySetInnerHTML={{ __html: JSON.stringify(schema) }}
+    />
+  );
+}
+
 // FAQ Schema for SEO
 function FAQSchema({ faq }: { faq: FAQItem[] }) {
   const schema = {
@@ -177,8 +216,8 @@ function ProductPage({ product }: { product: ProductCategory }) {
     <>
       {/* Schema.org Structured Data */}
       <ProductSchema product={product} />
+      <BreadcrumbSchema product={product} />
       {product.faq && product.faq.length > 0 && <FAQSchema faq={product.faq} />}
-      
       <div className="pt-24 pb-16">
         <div className="container mx-auto px-4">
           {/* Breadcrumb */}