Remove custom installCommand - let Vercel handle Python dependencies automatically

vercel.json

@@ -1,5 +1,4 @@
 {
   "buildCommand": "cd frontend && npm ci && npm run build",
-  "outputDirectory": "frontend/dist",
-  "installCommand": "pip install --upgrade pip && pip install -r requirements.txt"
+  "outputDirectory": "frontend/dist"
 }