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/static/_rpqMPjhAloA06YmSOFCm/_ssgManifest.js

@@ -0,0 +1 @@
+self.__SSG_MANIFEST=new Set,self.__SSG_MANIFEST_CB&&self.__SSG_MANIFEST_CB();