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/chunks/app/_not-found/page-a8311dbf26353b5f.js

@@ -0,0 +1 @@
+(self.webpackChunk_N_E=self.webpackChunk_N_E||[]).push([[409,746],{3521:function(){}},function(n){n.O(0,[971,23,744],function(){return n(n.s=3521)}),_N_E=n.O()}]);