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/emailtemplate_test.go

@@ -0,0 +1,52 @@
+package account
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestRenderConfirmationEmail checks that each (purpose, locale) renders a localised
+// subject, embeds the code and the one-tap deeplink in both bodies, and produces HTML.
+func TestRenderConfirmationEmail(t *testing.T) {
+	const deeplink = "https://erudit-game.ru/app/#/confirm/tok123"
+	cases := []struct {
+		name, purpose, locale, subjectSub string
+	}{
+		{"login ru", purposeLogin, "ru", "вход"},
+		{"login en", purposeLogin, "en", "sign-in"},
+		{"link ru", purposeLink, "ru", "подтвержд"},
+		{"link en", purposeLink, "en", "confirmation"},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			msg, err := renderConfirmationEmail(c.purpose, "123456", deeplink, "https://erudit-game.ru", c.locale)
+			if err != nil {
+				t.Fatalf("render: %v", err)
+			}
+			if !strings.Contains(strings.ToLower(msg.Subject), c.subjectSub) {
+				t.Errorf("subject %q does not contain %q", msg.Subject, c.subjectSub)
+			}
+			if !strings.Contains(msg.Text, "123456") || !strings.Contains(msg.HTML, "123456") {
+				t.Error("code missing from a body")
+			}
+			if !strings.Contains(msg.Text, deeplink) || !strings.Contains(msg.HTML, "confirm/tok123") {
+				t.Error("deeplink missing from a body")
+			}
+			if !strings.Contains(msg.HTML, "<html") {
+				t.Error("HTML body is not HTML")
+			}
+		})
+	}
+}
+
+// TestRenderConfirmationEmailUnknownLocaleDefaultsEnglish falls back to English for an
+// unsupported locale rather than erroring or emitting an empty subject.
+func TestRenderConfirmationEmailUnknownLocaleDefaultsEnglish(t *testing.T) {
+	msg, err := renderConfirmationEmail(purposeLogin, "000000", "", "https://erudit-game.ru", "de")
+	if err != nil {
+		t.Fatalf("render: %v", err)
+	}
+	if !strings.Contains(strings.ToLower(msg.Subject), "sign-in") {
+		t.Errorf("unknown locale should default to English, got subject %q", msg.Subject)
+	}
+}