feat(account): brand and harden the email pipeline

Swap the net/smtp mailer for go-mail behind the existing Mailer seam: the
Message struct now carries a text + HTML body, TLS mode is chosen from the port
(implicit TLS on 465, else mandatory STARTTLS), a dial timeout bounds the
synchronous send, and no client certificate is needed. Add a branded, image-free,
mobile-friendly ru/en HTML template (with a plain-text alternative) rendering a
large readable code and an ignore-notice footer with a landing link.

Add BACKEND_PUBLIC_BASE_URL config (the canonical origin for the email footer
link, never the request Host — anti-injection), required when a relay is
configured.

Fix the email-login address squat: ProvisionEmail creates the account flagged
is_guest until the code is confirmed, so an abandoned login is reaped like any
guest and its address freed; confirming (login or link) clears the flag. Seed the
new account's language from the client, plumbed through the email-login request.

The confirm deeplink, its transport surface and the send rate-limit land in
follow-up work.

backend/internal/account/link.go

@@ -26,16 +26,7 @@ func (s *EmailService) RequestLinkCode(ctx context.Context, accountID uuid.UUID,
 	if err != nil {
 		return err
 	}
-	code, hash, err := generateCode()
-	if err != nil {
-		return err
-	}
-	if err := s.store.replacePendingConfirmation(ctx, accountID, addr, hash, s.now().Add(emailCodeTTL)); err != nil {
-		return err
-	}
-	subject := "Your Scrabble confirmation code"
-	body := fmt.Sprintf("Your confirmation code is %s. It expires in %d minutes.", code, int(emailCodeTTL/time.Minute))
-	return s.mailer.Send(ctx, addr, subject, body)
+	return s.issueCode(ctx, accountID, addr, purposeLink, s.accountLocale(ctx, accountID))
 }
 
 // ConfirmLink verifies code for (accountID, email) and reports the address's