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/pages-manifest.json

@@ -0,0 +1,5 @@
+{
+  "/_app": "pages/_app.js",
+  "/_error": "pages/_error.js",
+  "/_document": "pages/_document.js"
+}