fix(hub): a desktop solve reports no hostname at all, not "meshbay"

Registering from the native client failed with `captcha_failed` while the
checkbox was green — a worse symptom than the one being fixed, because the
widget now looked fine and only the hub's own log said otherwise:

    captcha solved on an unexpected host ''; allowed: ['localhost', 'meshbay', 'meshbay.org']

The previous commit assumed Google would report the host component of the
origin, so `app://meshbay` would come back as `meshbay` and could sit in
`allowed_hosts`. It does not. A solve Google cannot attribute to a domain
reports an **empty** hostname, and no allowlist entry can match that. An empty
entry is not the answer either: a blank in a TOML list is a typo far more
often than an intention, and `load_config` drops blanks for that reason —
`captcha.allow_unattributed_host` is a named flag instead, so the trade is
stated where it is made.

What it admits, plainly: every non-web client, not only ours. A file:// page
or somebody else's Electron application look identical from here. That is the
same bar the client's own origin would have been — main.js already records
that `app://meshbay` is not a credential — and it is a bar: the captcha still
has to be solved, per token, in something that can render it. What is given up
is the origin restriction for non-web clients, not the captcha. Off by
default, and a hub without the desktop client should leave it off.

The refusal now names which of the two it is, since they need different
answers: an unexpected host names the host, an unattributed one says to set
the flag.

docs/captcha.md §6 said `meshbay` was the value and told operators to add it;
it now records what was measured and why the guess was wrong. The packaged
example config carries the flag with the same warning.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014UtzVrzM7e2tG9fSpkR9ML

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