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/ui/Button.tsx

@@ -0,0 +1,42 @@
+import { ButtonHTMLAttributes, forwardRef } from 'react';
+import { cn } from '@/lib/utils';
+
+interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: 'primary' | 'secondary' | 'outline' | 'ghost';
+  size?: 'sm' | 'md' | 'lg';
+}
+
+const Button = forwardRef<HTMLButtonElement, ButtonProps>(
+  ({ className, variant = 'primary', size = 'md', children, ...props }, ref) => {
+    return (
+      <button
+        ref={ref}
+        className={cn(
+          'inline-flex items-center justify-center font-medium transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary-500 disabled:pointer-events-none disabled:opacity-50',
+          // Variants
+          {
+            'bg-primary-600 text-white hover:bg-primary-700 active:bg-primary-800': variant === 'primary',
+            'bg-secondary-100 text-secondary-900 hover:bg-secondary-200': variant === 'secondary',
+            'border-2 border-primary-600 text-primary-600 hover:bg-primary-50': variant === 'outline',
+            'text-secondary-600 hover:bg-secondary-100': variant === 'ghost',
+          },
+          // Sizes
+          {
+            'px-3 py-1.5 text-sm rounded': size === 'sm',
+            'px-4 py-2 text-base rounded-md': size === 'md',
+            'px-6 py-3 text-lg rounded-lg': size === 'lg',
+          },
+          className
+        )}
+        {...props}
+      >
+        {children}
+      </button>
+    );
+  }
+);
+
+Button.displayName = 'Button';
+
+export { Button };
+export type { ButtonProps };