Fix common app-building errors

Plain-words fixes for the error messages beginners hit most when building with AI tools and coding agents.

• npm ERR! code ERESOLVE: what it means and how to fix it
  npm refuses to install because two packages want different versions of the same thing. Read the message, then pick the safe fix first.
• "command not found: npm" (or node): what it means and how to fix it
  Your terminal cannot find npm or node. Usually Node.js is not installed yet, or the terminal was opened before it was. Here is the order to check.
• "Missing environment variable" / API key not set: what to do
  The app works on your computer but says an environment variable or API key is missing once deployed. Add it to the host, then redeploy.
• "new row violates row-level security policy": what it means and the fix
  Supabase blocked your insert because no policy allows it. Add an insert policy with a check on who owns the row. Example included.
• "401 Unauthorized" / invalid API key from the Claude API: how to fix it
  A 401 from the Claude API means a problem with your API key. Check the key, where it is sent from, and whether it was revoked or expired.
• "blocked by CORS policy: No Access-Control-Allow-Origin header": the fix
  Your browser blocked a request to another site because the server did not say it is allowed. Fix it on the server, or route through your own backend.
• "Build failed: Module not found: Can't resolve": why it happens and the fix
  Your app builds on your computer but the deploy fails with Module not found. Usually a missing package, a wrong path or a capital letter.
• "Webhook signature verification failed" (Stripe, Dodo Payments): the fix
  Your payment webhook is rejected because the signature does not match. Check the signing secret and that you verify the raw, untouched body.
• "Error: listen EADDRINUSE: address already in use :::3000": the fix
  Your dev server cannot start because something is already using its port. Stop the old process or start on a different port.
• "Permission denied (publickey)" on git push to GitHub: the fix
  GitHub does not recognise your SSH key. Test the connection, load your key, and make sure its public half is on your account.

Bilo works by connecting to your own AI (Claude, ChatGPT, …). You need an account with that AI, and the usage for writing things comes out of your own AI plan.