Move the +/-1 window helper to internal/util/totp so both 2FA
acceptance points use it: login (CheckUser) and disable/rebind plus
username/password changes (VerifyTwoFactorCode). Also shrink comments
to the 2-line house rule and anchor the unit test mid-window to avoid
a step-boundary flake.
Addresses review on #6546 (MEDIUM + 2 LOWs).
sdhfsl