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, 37 insertions, 12 deletions

diff --git a/packages/meshbay-hub/tests/test_desktop_shell.py b/packages/meshbay-hub/tests/test_desktop_shell.py
index 13d194e..c4f5b08 100644
--- a/packages/meshbay-hub/tests/test_desktop_shell.py
+++ b/packages/meshbay-hub/tests/test_desktop_shell.py
@@ -85,11 +85,18 @@ def test_no_permission_is_granted_to_the_page():
 ])
 def test_the_scheme_is_privileged(privilege):
     """
-    Without `secure` the scheme is not a secure context, the service worker
-    silently refuses to register, and streamed downloads break with no error —
-    the same failure mode as an uncontrolled page, which this codebase has
-    already learned once. `standard` gives a real origin, so IndexedDB survives
-    an update instead of being keyed to something that moves.
+    `secure` is what makes it a secure context, and without it **the whole of
+    `crypto.subtle` is undefined** — measured, not assumed: the first probe
+    loaded a `data:` URL and every algorithm failed with TypeError, AES-GCM
+    included. `standard` gives a real origin, so IndexedDB survives an update
+    instead of being keyed to something that moves.
+
+    An earlier version of this docstring said `secure` was what let the service
+    worker register. That is wrong: Chromium refuses to register a worker on a
+    custom scheme whatever its privileges — "The URL protocol of the current
+    origin ('app://meshbay') is not supported". The application therefore has no
+    service worker and does not need one; it saves files through a native
+    dialog, which is better than the path the worker exists to provide.
     """
     assert privilege in _main(), f"{privilege} missing from the scheme privileges"
 
@@ -107,6 +114,18 @@ def test_the_protocol_handler_cannot_be_walked_out_of():
 
 # ── Content Security Policy ─────────────────────────────────────────────────
 
+def test_the_policy_is_sent_as_a_header():
+    """
+    A <meta> policy cannot carry `frame-ancestors`, and having it there means
+    one directive of the policy is decoration. The handler is also the only
+    thing that serves the interface, so this is one source rather than two.
+    """
+    source = _main()
+    assert "'Content-Security-Policy': CSP" in source
+    assert "Content-Security-Policy" not in INDEX.read_text(encoding="utf-8") \
+        .split("-->")[1], "the packaged page still carries a policy of its own"
+
+
 def test_the_policy_keeps_wasm_unsafe_eval():
     """
     The bundle key is Argon2id in WebAssembly. A policy that forbids it does not
@@ -116,14 +135,20 @@ def test_the_policy_keeps_wasm_unsafe_eval():
 
 
 def _policy() -> str:
-    """The meta tag's content, not the file — the comment above it names the
-    same directives and would satisfy a naive search."""
+    """
+    The policy the protocol handler sends, read out of the CSP constant.
+
+    Not a <meta> tag: `frame-ancestors` is ignored there, and a directive that
+    silently does nothing is worse than one that is absent. Chromium said so in
+    the console the first time the application was launched.
+    """
     import re
-    page = INDEX.read_text(encoding="utf-8")
-    match = re.search(
-        r'http-equiv="Content-Security-Policy"\s+content="([^"]*)"', page)
-    assert match, "no Content-Security-Policy meta tag"
-    return match.group(1)
+    source = _main()
+    match = re.search(r"const CSP = \[(.*?)\]\.join", source, re.S)
+    assert match, "no CSP constant in the main process"
+    return "; ".join(
+        line.strip().strip('",').strip('"')
+        for line in match.group(1).splitlines() if line.strip())
 
 
 def _directive(name: str) -> str: