Improve user registration and login security and data handling

Update SQL queries in auth and dashboard files to use prepared statements with placeholders (?) instead of direct variable interpolation ($1) for enhanced security and to prevent SQL injection vulnerabilities. Additionally, modify the user registration process to allow the database to auto-generate user IDs.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 81758ee2-4d29-44fe-aa0f-29b6f2ba15d4
Replit-Commit-Checkpoint-Type: intermediate_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/c1cf2404-8fb6-466d-b645-fe133323b8ed/81758ee2-4d29-44fe-aa0f-29b6f2ba15d4/NaOMok2
