Built-in Duplicate Email Handling with Customizable Email Templates

Problem Statement Currently, when users try to register with an existing email, Supabase returns a "successful" response with fake user data. I know this is for security reasons and it is valid, but since in our code we expected the user to confirm their account, we show a component that tells the user that registration was successful and they should check their email to confirm and log in, even if this is for security reasons, but it still creates several issues: 1. Poor User Experience: Users think registration succeeded but receive no confirmation email 2. Security Risk: Frontend can't distinguish between real and fake user responses 3. Database Constraints: Attempting to use fake user IDs causes foreign key violations 4. Confusion: Users don't know why they can't access their "new" account Current Supabase Behavior well we could easily fix the Security Risk and Database Constraints, by checking if the user exists in the databas…