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.

nixpacks.toml

@@ -0,0 +1,14 @@
+# Nixpacks Configuration for Astro
+# https://nixpacks.com/docs/configuration/file
+
+[phases.setup]
+nixPkgs = ["nodejs-20_x"]
+
+[phases.install]
+cmds = ["npm ci"]
+
+[phases.build]
+cmds = ["npm run build"]
+
+[start]
+cmd = "npm run preview -- --host 0.0.0.0 --port $PORT"