feat(mail): one layout for all mails with logo, greeting, cards and footer

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/s4_test.go

@@ -34,7 +34,7 @@ func TestDemoSendsNothingAndKeepsAccounts(t *testing.T) {
 	if err := srv.sendDiscordNotification("https://discord.com/api/webhooks/1/x", "x", "", "", ""); err != errDemoOutgoing {
 		t.Fatalf("discord sender must refuse in demo, got %v", err)
 	}
-	if err := srv.sendEmailNotification("a@example.com", "x", "x"); err != errDemoOutgoing {
+	if err := srv.sendMailContent("a@example.com", "x", mailContent{Intro: "x"}); err != errDemoOutgoing {
 		t.Fatalf("mail sender must refuse in demo, got %v", err)
 	}
 	blocked := []struct{ method, path, body string }{