feat: add settings defaults and validation with unit tests

package.json

@@ -1,7 +1,8 @@
 {
   "name": "search-with-chatgpt-powered-by-openai-extension",
-  "version": "1.0.0",
+  "version": "1.1.0",
   "private": true,
+  "type": "module",
   "description": "Development tooling for the Search with ChatGPT Firefox extension.",
   "engines": {
     "node": ">=22"
@@ -9,8 +10,9 @@
   "scripts": {
     "dev": "web-ext run",
     "lint": "web-ext lint",
+    "test:unit": "node --test",
     "build": "web-ext build",
-    "test": "npm run lint && npm run build"
+    "test": "npm run lint && npm run test:unit && npm run build"
   },
   "devDependencies": {
     "web-ext": "10.5.0"