fix(client): the desktop client runs, and running it corrected three things

Electron 42 / Chromium 148, launched under xvfb. The packaged interface mounts
over `app://` with a secure context, `crypto.subtle` present, Argon2 WASM
loaded, and no console errors. Three statements in the design were wrong, and
only launching it found them.

**A CSP in a `<meta>` tag silently drops `frame-ancestors`.** Chromium says so
in the console. A policy carrying a directive that does nothing is worse than
one without it, so the policy is sent as a header by the protocol handler —
which is also the only thing serving the interface, so one source instead of
two.

**`secure: true` is not what makes the service worker register.** Chromium
refuses a worker on a custom scheme whatever its privileges: "The URL protocol
of the current origin ('app://meshbay') is not supported". The application has
no service worker and needs none — it saves through a native dialog, which is
the better of the two paths. `sw.js` stays in the package because the same files
serve the browser, where it is one of only three ways to write a large file.

What `secure: true` is actually for was measured at the same time: without it
**the whole of `crypto.subtle` is undefined**. The first probe loaded a `data:`
URL and every algorithm failed with TypeError, AES-GCM included — which is why
the probe was rewritten before believing its answer. X25519 and Ed25519 are both
present on Chromium 148, settling the version floor left open as O6.

**The renderer cannot call the hub.** Its origin is `app://meshbay` and CORS
refuses it. The hub has *no CORS middleware at all* — its API is reachable from
no web origin whatever — and that is worth keeping. Widening it for
`app://meshbay` would be worse than it looks: that origin is not a credential,
since any Electron application can claim the same scheme and host name.

So every hub call leaves from the main process, exactly as saving a file does,
and it refuses any origin that is not the hub the user signed in to.
`platform.apiFetch()` is `fetch` in a browser and the bridge in the application,
so no caller has to know which it got. `transport.js` reaches it through a
global because it is a classic script, not a module — the alternative was a
second fetch path, which is how two callers of one hub start disagreeing about
how to reach it.

Verified from inside Electron: the main process gets 200 from
/v1/hub/version, the renderer is refused by CORS, and **a script served by the
hub is refused by the policy** — T3's mitigation demonstrated rather than
asserted.

Build note, written into the README because it will bite the next person:
**Ubuntu 24.04's nodejs 18 cannot install Electron at all** — the download
script `require()`s an ESM module, which Node gained in 22. Node 24 LTS,
checksum-verified against nodejs.org, is what this was built with.
package-lock.json is committed; builds use `npm ci`, not `npm install`.

799 tests pass, e2e.py still passes end to end. The session harness needed a
platform stub: it lifts `hubFetch` out of app.js as text and runs it, so the
adapter is now part of the environment it models.

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

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