* fix(panel): accept 2FA codes from adjacent TOTP windows
CheckUser compared only gotp.Now(), so a code submitted at the end of
its 30s window (or with slight client/server clock drift) failed with
'invalid 2fa code', while the immediate retry in the next window
succeeded. Accept current +/-1 window, the standard TOTP skew
tolerance.
Fixes MHSanaei/3x-ui#6535
* fix(panel): share TOTP skew tolerance with VerifyTwoFactorCode
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).
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Co-authored-by: sdhfsl <sdhfsl@users.noreply.github.com>
sdhfsl