Use @astrojs/sitemap instead of manual sitemap.xml.ts

- astro.config.mjs: site=https://dealplustech.co.th, add sitemap integration
- Remove hardcoded sitemap.xml.ts (now auto-generated)

package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "@astrojs/check": "^0.9.4",
     "@astrojs/react": "^5.0.5",
+    "@astrojs/sitemap": "^3.7.3",
     "@tailwindcss/typography": "^0.5.15",
     "@tailwindcss/vite": "^4.0.0",
     "astro": "^6.1.7",