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)

astro.config.mjs

@@ -1,13 +1,14 @@
 import { defineConfig } from 'astro/config'
 import tailwindcss from '@tailwindcss/vite'
 import react from '@astrojs/react'
+import sitemap from '@astrojs/sitemap'
 import { fileURLToPath } from 'url'
 import path from 'path'
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url))
 
 export default defineConfig({
-  site: 'https://dealplustech.com',
+  site: 'https://dealplustech.co.th',
   output: 'static',
   vite: {
     plugins: [tailwindcss()],
@@ -22,6 +23,7 @@ export default defineConfig({
   },
   integrations: [
     react(),
+    sitemap(),
   ],
   build: {
     assets: '_assets',