feat: add Vitest configuration and tests for lesson modules and rendering

package.json

@@ -8,7 +8,11 @@
     "dev": "vite",
     "build": "vite build",
     "preview": "vite preview",
-    "test": "vitest"
+    "test": "vitest run",
+    "test.watch": "vitest watch",
+    "test.coverage": "vitest run --coverage",
+    "format": "prettier --write src/ tests/ package.json vite.config.js vitest.config.js",
+    "format.lessons": "prettier --write lessons/*.json"
   },
   "keywords": [
     "css",
@@ -20,7 +24,15 @@
   "author": "Michael Czechowski <mail@dailysh.it>",
   "license": "Copyright Michael Czechowski 2025",
   "devDependencies": {
+    "@testing-library/dom": "^10.4.0",
+    "@testing-library/jest-dom": "^6.6.3",
+    "@vitest/coverage-v8": "^3.1.3",
+    "jsdom": "^26.1.0",
+    "prettier": "^3.5.3",
     "vite": "^6.3.5",
     "vitest": "^3.1.3"
+  },
+  "dependencies": {
+    "whatwg-fetch": "^3.6.20"
   }
 }