chore: migrate from npm to pnpm

Replace package-lock.json with pnpm-lock.yaml and add packageManager field.

package.json

@@ -1,6 +1,7 @@
 {
   "name": "exchange-rate-export",
   "version": "0.1.0",
+  "packageManager": "pnpm@11.1.1",
   "private": true,
   "scripts": {
     "dev": "next dev --turbopack",
@@ -26,5 +27,8 @@
     "eslint": "^9",
     "eslint-config-next": "15.3.4",
     "tailwindcss": "^4"
+  },
+  "pnpm": {
+    "ignoredBuiltDependencies": ["sharp", "unrs-resolver"]
   }
 }