Add two-factor login (TOTP) with recovery codes

Users can set up an authenticator app from their account page; login
then asks for a code after the password, with one-time recovery codes
as a fallback. Sensitive actions (tokens, user changes, 2FA resets)
need a code from the last 5 minutes. totp.require (none/admins/all)
forces setup, and admins can reset a user's 2FA from the UI, the jarl
CLI or jarld on the server. The dashboard and jarl status flag users
missing 2FA and a shared "admin" account still in use.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go.sum

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+rsc.io/qr v0.2.0 h1:6vBLea5/NRMVTz8V66gipeLycZMl/+UlFmk8DvqQ6WY=
+rsc.io/qr v0.2.0/go.mod h1:IF+uZjkb9fqyeF/4tlBoynqmQxUoPfWEKh921coOuXs=