codeSplitters: turn on account email live (onboard a sender domain) #276

Follow-up from #253. Verification and password reset mail are in the code but off on the live site: codesplitters runs on workers.dev, and Cloudflare Email Sending needs a sender domain onboarded (wrangler email sending enable <domain>, Workers Paid: 3,000 emails a month included, then $0.35 per 1k).

To turn it on: onboard a domain, then deploy with CODESPLITTERS_MAIL_FROM=accounts@<domain> (rustybuns.config.ts binds EMAIL and EMAIL_FROM from it). Until then, email accounts can't verify or reset, and grants still need GitHub or Google sign-in.