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.

dealplustech-astro/.vscode/launch.json

@@ -0,0 +1,11 @@
+{
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "command": "./node_modules/.bin/astro dev",
+      "name": "Development server",
+      "request": "launch",
+      "type": "node-terminal"
+    }
+  ]
+}