refactor: Move Astro project to root directory

- Move all Astro files from dealplustech-astro/ to root
- Archive Next.js code in _nextjs-backup/
- Update .gitignore for Astro project
- Simplify project structure

This completes the migration from Next.js to Astro.
The Astro project is now at the root level.

.next/server/app/blog/[slug]/page.js.nft.json

@@ -0,0 +1 @@
+{"version":1,"files":["../../../webpack-runtime.js","../../../chunks/948.js","../../../chunks/12.js","../../../chunks/673.js","../../../chunks/904.js","page_client-reference-manifest.js","../../../../../package.json","../../../../../src/content/blog/ข้อดี-ท่อ-hdpe.md","../../../../../src/content/blog/บำรุงรักษาปั๊มน้ำ.md","../../../../../src/content/blog/ท่อ-ppr-คืออะไร.md"]}

.next/server/app/blog/page.js.nft.json

@@ -0,0 +1 @@
+{"version":1,"files":["../../webpack-runtime.js","../../chunks/948.js","../../chunks/12.js","../../chunks/673.js","../../chunks/904.js","page_client-reference-manifest.js","../../../../package.json","../../../../src/content/blog/ข้อดี-ท่อ-hdpe.md","../../../../src/content/blog/บำรุงรักษาปั๊มน้ำ.md","../../../../src/content/blog/ท่อ-ppr-คืออะไร.md"]}