fix(node): report the transfer cap the node actually enforces

`transfer_state` read `slots.per_member` — the node-wide default — while
`_has_room` decides with `member_cap()`, which prefers the group's own signed
limit, and the handshake ack announces that same `member_cap()`. Three readings
of one number, and one of them was the odd one out.

In a group where the operator signed a higher limit, every lease update told the
client "cap: 2" while the node would grant five: the transfers widget draws
`used >= cap` as saturated, so a member with two transfers running saw the rest
of their slots disappear. Lowered the other way it is worse in the other
direction — the interface offers slots the node will queue.

Nothing was ever granted or refused wrongly; the enforcement was right on both
paths. It is the number beside it that contradicted them.

Two tests, one override above the default and one below, because a bug that
reads the node-wide value passes the first whenever the default happens to be
the larger number.

Node suite 1215 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

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