Refactor: Update category structure, mega menu, footer, and remove unused pages

- Move DURGO from ระบบน้ำ to อุปกรณ์ปรับอากาศ
- Add -category suffix to อุปกรณ์ดับเพลิง and ฉนวนหุ้มท่อ category pages
- Update Header mega menu with correct category slugs
- Fix Footer layout: split categories to 2 columns, remove quick links
- Delete unused pages: all-projects, join-us, services, product
- All product images fixed to 1:1 aspect ratio

public/robots.txt

@@ -0,0 +1,11 @@
+User-agent: *
+Allow: /
+
+# Disallow admin areas
+Disallow: /admin/
+
+# Sitemap location
+Sitemap: https://dealplustech.co.th/sitemap-index.xml
+
+# Crawl delay (optional, be nice to small servers)
+Crawl-delay: 1