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/_document.js.nft.json

@@ -0,0 +1 @@
+{"version":1,"files":["../webpack-runtime.js","../chunks/682.js","../../../package.json","../../../node_modules/next/dist/pages/_document.js"]}