test(account): unlink guard + change-email replace/refuse/deeplink

Integration: unlinking a provider keeps the other identities and refuses removing
the last one; change-email replaces the address (freeing the old), refuses a taken
address without merging, and works through the one-tap deeplink token. Unit: the
change-email template renders localised ru/en copy.

backend/internal/account/emailtemplate_test.go

@@ -16,6 +16,8 @@ func TestRenderConfirmationEmail(t *testing.T) {
 		{"login en", purposeLogin, "en", "sign-in"},
 		{"link ru", purposeLink, "ru", "подтвержд"},
 		{"link en", purposeLink, "en", "confirmation"},
+		{"change ru", purposeChange, "ru", "новый"},
+		{"change en", purposeChange, "en", "new"},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {