fix(node): the node's own controls take no authority from a hub token

`_is_node_admin()` is `self._user_id == node_user_id`, and `_user_id` is the
`sub` of a JWT the hub issued. Six node-wide controls were gated on that alone:
`node_status` — which lists every group on the machine with each root's
**absolute path** — plus `node_settings_set`, `roster_read`, `denylist_read`,
`denylist_clear` and `node_reload`.

So the answer to "are you the operator of this node" was "the hub says so",
which NS4 and M3 rule out in as many words: operator authority comes from the
node's roster and from nowhere else, and asking the hub is how the hub installs
itself as node administrator. The reach is bounded — a completed handshake also
needs the group key — but an active hub obtains one legitimately in an
open-join group, which §3.5 concedes, and from there it could read the
operator's directory layout or clear the denylist, which is the persisted
revocation H4 exists to keep.

`_operator_device()` requires both halves now: the account is the one the node
belongs to, *and* the device on this connection has proved a key the roster
holds as an operator. `device_hello` is signed over a transcript naming the
node, the group and this connection's nonce, and `operator_pks()` is rebuilt
from the roster on each call, so an unpinned browser and a revoked one are both
refused at once. The hub holds no user keys and cannot countersign a device.

Keeping the account check as well is deliberate: dropping it would widen these
node-wide controls to any paired operator of any group on the machine, which is
a separate decision. `_is_node_admin()` stays as what it is in the handshake
ack — a hint telling a client whether to offer the Node page — and says so.

Nothing changes for a paired operator: `device_hello` runs unconditionally
after the ack, and anyone using the Node page's controls is already paired,
since `root_add` and every other signed op has always verified against
`operator_pks()`. A browser that never paired now reads nothing there, which is
the state in which it could already write nothing.

test_node_status.py's fixture set the account and not the device, which is how
it went on passing; it now wires the device the way `device_hello` leaves it.
The adversary itself is in test_security_regressions.py — a token naming the
owner's account with no proved device, which the previous source answered with
`node_status_ack`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UMxEQadpzPkYLFf5CYKhpW

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