fix(hub): check the captcha's origin here, so the desktop client can pass one

Reported from the native client: the reCAPTCHA box renders
"ERROR for site owner: Invalid domain for site key". The web browser is fine.

It is not a client restriction, and the CSP was never what refused — the
script loads, which is why the widget appears at all to say so. reCAPTCHA
validates the hostname of the page the widget is rendered in against the
domain list on the site key, and the desktop client's interface ships inside
the package and is served from `app://meshbay` (main.js: `win.loadURL`). Not a
preference: file:// breaks ES modules and IndexedDB, and the hub must never
become the document origin. So the hostname Google sees is `meshbay`, it is
not on the key's list, and it never can be — the check runs on Google's
servers and nothing client-side reaches it.

The fix turns that check off on the key and does it on the hub instead:

    [captcha]
    allowed_hosts = ["meshbay.org", "localhost", "meshbay"]

`verify_captcha` refuses a solve whose hostname is not in the list. The
hostname comes from `siteverify` — what Google observed, not what the caller
asserts — so it is a real check against what turning the console setting off
opens, which is a bot rendering the public site key on a page of its own.
Empty (the default) skips it, so an existing hub upgrades unchanged with
reCAPTCHA still doing the origin check. The two settings go together, and
docs/captcha.md §6 says so.

The `meshbay` entry is the weak one and the doc says that too: any Electron
application can claim the same scheme and host, as main.js already records.
What it still costs is a captcha solve per token inside a real Chromium
instead of a token farmed from any web page.

docs/captcha.md §6 replaced. It documented a design that was superseded twice
— an `auth_key`-keyed carve-out that turned out to disable the gate for
everyone, and "works in the Electron client too, both run Chromium", which is
the assumption this bug is made of: reCAPTCHA validates the domain, not the
rendering engine.

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

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