Update TUTORIAL.md for the dual request/approve onboarding mechanism
Documents the optional-password behavior on all three onboarding forms
(blank = pending request, correct = instant activation, wrong =
rejected outright), the new /admin/pending review page, and the third
dashboard button (+ Add Radicle peer) added alongside it. Also
corrects an inaccuracy the rewrite would otherwise have kept: the
share-claim passcode was described as an admin_password check: it
isn't one, it's a separate secret verified by AES-GCM decryption, not
a stored comparison.