Enhance development and production scripts in package.json and update README for CSP testing

- Added new npm scripts for production testing with Content Security Policy: `dev:prod` and `preview:prod`.
- Updated README.md to include detailed instructions for development, production testing, and a pre-deployment checklist to ensure security settings are verified before deployment.

package.json

@@ -9,8 +9,10 @@
   },
   "scripts": {
     "dev": "astro dev",
+    "dev:prod": "npm run build && ENABLE_SSR_CSP=1 npm run start",
     "build": "astro build",
     "preview": "astro preview",
+    "preview:prod": "ENABLE_SSR_CSP=1 npm run start",
     "start": "node server.js",
     "astro": "astro",
     "check": "npm run check:astro && npm run check:eslint && npm run check:prettier",