Full Stack Developer for Customer Portal

Hi, We're a small HVAC service company. About 6 months ago a freelancer started building a customer portal for us in Next.js with a Node/Express backend and Postgres. He got maybe 60% done and then stopped replying, so the code is there but some parts only half work. What needs to happen first: - login is buggy, customers sometimes get kicked out right after signing in (I think it's something with sessions) - booking page isn't finished, customers should pick a time slot and it should land in our Google Calendar - invoice history page, the data is already in the database, just needs to be shown Then the bigger part. We have around 300 PDFs, equipment manuals plus our own service notes. I'd like a chat inside the portal where techs can ask things like "what does error E4 mean on a Carrier unit" and get an answer from our own docs, not generic stuff. Someone told me this can be done with OpenAI but I don't know what the right way is or what it costs to run monthly. Code is on GitHub, I'll share access after a short call. Hosting is on Vercel. In your proposal please tell me how you'd approach the AI chat and roughly how long the first three fixes would take. If you've built something similar, send a link. Budget is around $4000 for everything, open to discuss if you can explain why it should be different. Want to start next week.