A configurable logo URL renders as an image beside the brand name in the
email header (falling back to the brand text when unset). Email clients
need a hosted image, so it takes a URL rather than an inline asset.
Redesigns the mail template (branded header bar, hero code block, copy
button, muted footer) and adds a configurable accent colour alongside the
brand name.
Emails now render an HTML template from templates/email with a plaintext
fallback, sent as multipart/alternative. A configurable brand name shows
in the template. Reset and confirmation mails share one styled base.
