move to yarn, add prettier

package.json

@@ -3,10 +3,13 @@
   "version": "0.1.0",
   "private": true,
   "dependencies": {
+    "@fontsource/roboto-mono": "^5.2.5",
     "@testing-library/dom": "^10.4.0",
     "@testing-library/jest-dom": "^6.6.3",
     "@testing-library/react": "^16.2.0",
     "@testing-library/user-event": "^13.5.0",
+    "date-fns": "^4.1.0",
+    "prettier": "^3.5.3",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "react-scripts": "5.0.1",
@@ -16,7 +19,8 @@
     "start": "react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test",
-    "eject": "react-scripts eject"
+    "eject": "react-scripts eject",
+    "lint": "prettier --write ."
   },
   "eslintConfig": {
     "extends": [
@@ -35,5 +39,6 @@
       "last 1 firefox version",
       "last 1 safari version"
     ]
-  }
+  },
+  "packageManager": "yarn@4.8.1"
 }