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>

1 files changed, 43 insertions, 0 deletions

diff --git a/packages/meshbay-client/README.md b/packages/meshbay-client/README.md
new file mode 100644
index 0000000..46be72b
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+++ b/packages/meshbay-client/README.md
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+# MeshBay desktop client
+
+The interface ships **inside this package** and loads from disk. A shell that
+points a WebView at the hub's `/app/` is a browser with a different icon and
+fixes nothing — that is finding T3, and removing it is the reason this exists.
+
+## Building
+
+Needs **Node ≥ 22**. Ubuntu 24.04 ships nodejs 18, which cannot install Electron
+at all: the download script `require()`s an ESM module, which Node gained only in
+22. Fedora 44 is fine.
+
+    # once, on a machine whose distro is too old:
+    curl -fsSLO https://nodejs.org/dist/v24.19.0/node-v24.19.0-linux-x64.tar.xz
+    curl -fsSLO https://nodejs.org/dist/v24.19.0/SHASUMS256.txt
+    grep " node-v24.19.0-linux-x64.tar.xz$" SHASUMS256.txt | sha256sum -c -
+    sudo tar -xJf node-v24.19.0-linux-x64.tar.xz -C /opt/nodejs --strip-components=1
+    export PATH=/opt/nodejs/bin:$PATH
+
+    npm ci            # not `npm install` — the lockfile is the build input
+    npm run sync-ui   # copy the interface from the hub package
+    npm start
+
+`ui/` is generated and is not committed. The interface has exactly one source,
+`packages/meshbay-hub/src/meshbay_hub/static/`, and a silent fork is the only
+real way to end up maintaining it twice.
+
+## Running headless
+
+    xvfb-run -a ./node_modules/electron/dist/electron --no-sandbox .
+
+## What it does not have
+
+**No service worker.** Chromium refuses to register one on a custom scheme, so
+the streamed-download path never runs here — the application saves through a
+native dialog instead, which is the better of the two. `sw.js` is still shipped
+because the same files serve the browser, where it is one of only three ways to
+write a large file.
+
+**No direct network from the renderer.** Its origin is `app://meshbay`, which
+CORS refuses, and the hub has no CORS middleware at all — a posture worth
+keeping. Every hub call leaves from the main process, which refuses any origin
+that is not the hub the user signed in to.