use vercel-preparation.sh also for frontend

frontend/vercel.json

@@ -1,5 +1,5 @@
 {
   "buildCommand": "yarn build-trends",
   "outputDirectory": "build",
-  "installCommand": "cp -r ../../backend/ ./src/backend/ && yarn install"
+  "installCommand": "chmod +x ../vercel-preparation.sh && ../vercel-preparation.sh && yarn install"
 }

vercel-preparation.sh

@@ -4,4 +4,4 @@ cd "$(dirname "$0")" || exit 1
 mkdir --parents backend/.vercel/output/functions/api.func/
 cp --recursive backend/* backend/.vercel/output/functions/api.func/
 cp --recursive backend/.vercel/output/functions/api.func/_dot_vercel_copy/output/ backend/.vercel/
-cp --recursive backend frontend/frontend/src/backend/
+cp --recursive backend frontend/frontend/src/backend/