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

public/robots.txt

@@ -0,0 +1,25 @@
+# Robots.txt for dealplustech.co.th
+# https://dealplustech.co.th
+
+User-agent: *
+Allow: /
+
+# Sitemap location
+Sitemap: https://dealplustech.co.th/sitemap.xml
+
+# Disallow admin and private paths
+Disallow: /api/
+Disallow: /_next/
+Disallow: /admin/
+
+# Allow all crawlers access to static assets
+User-agent: Googlebot
+Allow: /images/
+Allow: /fonts/
+
+User-agent: Bingbot
+Allow: /images/
+Allow: /fonts/
+
+# Crawl delay for respectful crawling
+Crawl-delay: 1