test: prove a second browser works after pairing

The mechanism was already there — the encrypted keypair bundle goes to the node
after a first successful connection, and any client holding the password can
recover it — but nothing exercised it. e2e.py never pushed a bundle, so the case
that matters to an ordinary user was the one case never tested.

It now does what app.js does: backs the member's keys up to the node, then opens
a second client carrying nothing but a username and a password. Against the live
deployment that client recovers its identity keys, is recognised as the same
person with no second code, gets the same group key, and browses the group.

Also guards the ordering this depends on: the keypair bundle must be fetched
before joinGroup() runs, or a browser that did not register has no key to sign
the join with — invisible on the browser that did register, broken on every
other one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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