Adds a "Trust this device for 30 days" checkbox to the 2FA challenge step,
and a "Trusted devices" section to the 2FA settings card: list devices with
a "this device" badge, per-row revoke, and a confirm-gated "sign out all
other trusted devices" action. Both flows are opt-in and additive -- default
unchecked, so nothing changes for a user who never uses them.
cesnimda