fix(ui): localise the confirm screen, drop the brand on the error state

The session-less /confirm page defaulted to English, so it showed the English
app title. Carry the recipient's language on the deeplink (?lang) and setLocale on
load so the page matches the email. Show a localised brand wordmark (Эрудит / Erudit,
matching the email) on the success state only; the invalid/expired state now shows
just the message, no header.

backend/internal/account/email.go

@@ -114,7 +114,7 @@ func (s *EmailService) issueCode(ctx context.Context, accountID uuid.UUID, email
 	if err := s.store.replacePendingConfirmation(ctx, accountID, email, codeHash, tokenHash, purpose, s.now().Add(emailCodeTTL)); err != nil {
 		return err
 	}
-	msg, err := renderConfirmationEmail(purpose, code, s.confirmURL(token), s.baseURL, locale)
+	msg, err := renderConfirmationEmail(purpose, code, s.confirmURL(token, locale), s.baseURL, locale)
 	if err != nil {
 		return err
 	}
@@ -122,13 +122,14 @@ func (s *EmailService) issueCode(ctx context.Context, accountID uuid.UUID, email
 	return s.mailer.Send(ctx, msg)
 }
 
-// confirmURL builds the absolute one-tap confirm deeplink for token, or "" when no
-// public base URL is configured.
-func (s *EmailService) confirmURL(token string) string {
+// confirmURL builds the absolute one-tap confirm deeplink for token in locale, or ""
+// when no public base URL is configured. The locale rides the fragment as ?lang so the
+// confirm screen (opened in a browser with no session) renders in the email's language.
+func (s *EmailService) confirmURL(token, locale string) string {
 	if s.baseURL == "" {
 		return ""
 	}
-	return strings.TrimRight(s.baseURL, "/") + emailConfirmPath + token
+	return strings.TrimRight(s.baseURL, "/") + emailConfirmPath + token + "?lang=" + normalizeLocale(locale)
 }
 
 // accountLocale returns the account's preferred UI language for localising email,