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/inttest/merge_test.go

@@ -119,7 +119,7 @@ func seatGame(t *testing.T, seats []uuid.UUID, timeout time.Duration) uuid.UUID
 
 func newLinkService(mailer account.Mailer) *link.Service {
 	store := account.NewStore(testDB)
-	emails := account.NewEmailService(store, mailer)
+	emails := account.NewEmailService(store, mailer, "https://erudit-game.ru")
 	sessions := session.NewService(session.NewStore(testDB), session.NewCache())
 	return link.NewService(emails, store, accountmerge.NewMerger(testDB), sessions)
 }