Migrate to @supabase/ssr package

[neatonk]

This should save some time by allowing me to leverage knowledge and recent experience with the ssr package.

See https://supabase.com/docs/guides/auth/server-side/migrating-to-ssr-from-auth-helpers

[neatonk]

• https://supabase.com/docs/guides/auth/server-side/email-based-auth-with-pkce-flow-for-ssr
• https://supabase.com/docs/guides/cli/customizing-email-templates