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/components/analytics/GoogleAnalytics.tsx

@@ -0,0 +1,31 @@
+'use client';
+
+import Script from 'next/script';
+
+const GA_MEASUREMENT_ID = process.env.NEXT_PUBLIC_GA_MEASUREMENT_ID;
+
+export default function GoogleAnalytics() {
+  if (!GA_MEASUREMENT_ID || GA_MEASUREMENT_ID === 'G-XXXXXXXXXX') {
+    return null;
+  }
+
+  return (
+    <>
+      <Script
+        src={`https://www.googletagmanager.com/gtag/js?id=${GA_MEASUREMENT_ID}`}
+        strategy="afterInteractive"
+      />
+      <Script id="google-analytics" strategy="afterInteractive">
+        {`
+          window.dataLayer = window.dataLayer || [];
+          function gtag(){dataLayer.push(arguments);}
+          gtag('js', new Date());
+          gtag('config', '${GA_MEASUREMENT_ID}', {
+            page_title: document.title,
+            page_location: window.location.href,
+          });
+        `}
+      </Script>
+    </>
+  );
+}