Send a Welcome Email After Signup

When a new user completes signup — whether via email/password or OAuth — trigger a welcome email using the Resend API. The email should address the user by their first name in the subject line ("Welcome, [Name] — you're in.") and in the opening line of the body. The body should include: a brief 3-sentence description of what the app does and the core problem it solves, a single prominent CTA button labeled "Get Started →" linking to the app's main dashboard URL, and a short PS line at the bottom with a link to the help documentation ("PS — if you ever get stuck, our help docs are here."). Style the email with a white background, the app's primary brand color for the button, and a clean single-column layout. Send the email from a no-reply@[yourdomain.com] address and set up Resend with the correct API key from environment variables.