fix(client): three defects a real desktop found in ten minutes

All three came from the operator running the application on Ubuntu GNOME. None
would have been found by anything already in the suite.

**A second copy of the hub address.** `keyderive.js` carried
`const HUB = '' // same origin` — true of a page the hub served, false of one
loaded from a package, where the origin is `app://meshbay` and `/v1/users/register`
resolves against the application's own protocol handler. **Sign-up and sign-in,
the first two things anybody does, failed with "Not found."** The seam was
changed in `app.js` and in the signalling call and this was missed: the same
shape as the duplicate `MNP_VERSION` in `protocol.py`, a second copy of a
constant that is harmless until the context changes. `test_hub_address_seam.py`
refuses any file that decides where the hub is, and any `fetch('/v1/…')`
relative to the page origin.

**A window handler reading a variable another path reassigns.** Changing the hub
closes one window and opens another; `closed` arrives *after* the replacement is
assigned, so the outgoing window nulled the reference to the incoming one and
its `ready-to-show` crashed on it — a modal "A JavaScript error occurred in the
main process". Every handler now belongs to the window it was created with. The
CDP test wrote `config.json` in advance, so it never took the one path that
creates a second window; it does now, starting from an empty user-data dir.

**A first run that could not be undone.** The hub address was accepted on
anything URL-shaped and there was no way to change it afterwards — the prompt
only appears when none is set, so a typo meant editing JSON by hand. `https`
typed at a hub speaking `http` produced `TypeError: fetch failed`, which names
nothing. Now: the address is probed before being written, failures say which
URL and why ("does not speak https. If this hub is on your own machine, it is
probably http"), Settings can change it, and Electron's
"Error invoking remote method" wrapper is stripped from what a person reads.

Verified on the operator's desktop: **safeStorage really uses the GNOME
keyring** — Settings reports `gnome-libsecret`, and `secrets.bin` is written
0600 with Chromium's `v11` prefix, the marker for keyring-backed encryption
(the fixed-key fallback writes `v10`). Headless, the same code reports
`unavailable` and refuses to store rather than downgrading in silence, which is
now explained in Settings instead of shown as a bare word.

Unrelated but found while testing: `test_locales.py` assigned to
`globalThis.navigator`, which is read-only from Node 22. The client's build
already requires Node 22+, so the first CI machine configured for it would have
failed these tests for no visible reason. 809 tests pass on Node 18 and Node 24.

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

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