fix(mail-index): real end-to-end verification, and the three bugs it found

Adds integration/tests/12-electron-mail-index.spec.ts (3 tests, all passing
against the real Stalwart fixture) and fixes what running it exposed. None of
these were visible from reading the code.

1. JMAP session fetch never followed a redirect. Stalwart 307-redirects
   /.well-known/jmap to /jmap/session, and fetchJmapSession used
   `redirect: 'manual'` and treated any non-2xx as failure - so every reindex
   died with "JMAP session fetch failed (307)". Now follows up to 3 hops and
   REFUSES to follow off-origin, because the user's credentials ride on every
   hop; a blind `redirect: 'follow'` would hand the Authorization header to
   whatever host a misconfigured session pointed at. Same bound and same
   reasoning as lib/auth/verify-jmap-auth.ts.

2. The fd-3 key channel could only be adopted once per process, but its state
   was module-scoped. Next re-evaluates route modules, so a second instance hit
   `Could not open fd 3: Error: open EEXIST` from libuv. State moved to a
   Symbol on globalThis - the one place in a Node process that survives module
   re-evaluation.

3. Next's output file tracing does NOT carry @signalapp/sqlcipher's prebuilds/
   into .next/standalone. It traced the package's JS and its node-gyp-build
   dependency, but node-gyp-build resolves the .node binary by scanning a
   directory at runtime, which no static tracer can follow - so `require()`
   would have failed in every packaged build. scripts/assemble-standalone.mjs
   now copies it, alongside the public/ and .next/static copies it already does
   for the same "standalone output omits things" reason. All six platform/arch
   prebuilds are copied, not just this host's, because electron-builder
   cross-builds the x64 and arm64 macOS targets from one runner.

The three tests, and why it takes three - two constraints made a single
configuration impossible, and both were measured rather than assumed:

  * The renderer cannot reach this fixture from a production build. Its CSP
    pins connect-src to `'self' https: wss:` and the fixture's Stalwart is
    plain HTTP. NODE_ENV=development at RUNTIME does not help: `next build`
    INLINES process.env.NODE_ENV into the compiled middleware, so proxy.ts's
    `isDev` is frozen at build time (observed: a standalone server started with
    NODE_ENV=development still served the production CSP).
  * The fd-3 channel cannot survive `next dev`, which forks its server with an
    IPC channel that claims fd 3 (EEXIST); fd 4 there is not a pipe either
    (ENOTTY).

  So: PIPELINE drives the real standalone server over HTTP from Node with a
  real fd-3 key channel (no browser, so no CSP) and asserts a real SMTP
  delivery is findable by a word from its BODY, with a real snippet and
  contextBlock, idempotent catch-up, working type filters, and - reading the
  raw bytes of the .db AND its -wal - that nothing is recoverable in cleartext.
  TRIGGER proves the event-driven wiring: a real delivery makes the renderer
  POST /api/offline/reindex off its live push. WIRING launches the real shell
  with no ELECTRON_LOAD_URL and asserts the routes are reachable (401, not 404
  or 503) with real safeStorage behind them.

Each test now gets its own --user-data-dir. That is load-bearing, not hygiene:
Electron reuses one profile across launches, and a leftover jmap_stalwart_ctx
cookie from an earlier run made the WIRING test's 401 assertion pass as a 200.

Verified: typecheck clean; unit suite 2379 tests with the SAME 3 pre-existing
failures as the base commit b15098a6 (2 builtin-themes, 1 jmap-client-
resilience) and 48 net new passing; both `docker build`s succeed; the
hosted-deployment gate returns 404 with an empty body and materialises no file
in the production image; e2e/electron-smoke 4/4; 11-electron-notification
still passes.

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

lib/mail-index/jmap.ts

@@ -75,14 +75,45 @@ async function fetchWithTimeout(url: string, init: RequestInit): Promise<Respons
   }
 }
 
+/** Stalwart 307-redirects /.well-known/jmap to /jmap/session. */
+const MAX_REDIRECTS = 3;
+
 export async function fetchJmapSession(serverUrl: string, authHeader: string): Promise<JmapSessionInfo> {
-  const response = await fetchWithTimeout(`${serverUrl.replace(/\/+$/, '')}/.well-known/jmap`, {
-    method: 'GET',
-    headers: { Authorization: authHeader },
-  });
+  const base = serverUrl.replace(/\/+$/, '');
+  const origin = new URL(base).origin;
+  let currentUrl = `${base}/.well-known/jmap`;
+  let response: Response | undefined;
+
+  // Redirects must be followed EXPLICITLY, not with `redirect: 'follow'`: we
+  // attach the user's credentials to every hop, so each one has to be checked to
+  // still be on the origin we authenticated against. A blind follow would hand
+  // the Authorization header to whatever host a misconfigured or hostile session
+  // pointed at. Same reasoning (and same bound) as lib/auth/verify-jmap-auth.ts.
+  for (let hop = 0; hop <= MAX_REDIRECTS; hop++) {
+    response = await fetchWithTimeout(currentUrl, {
+      method: 'GET',
+      headers: { Authorization: authHeader },
+    });
+    if (response.status < 300 || response.status >= 400) break;
+
+    const location = response.headers.get('location');
+    if (!location) throw new JmapIndexError('JMAP session redirect had no Location header');
+    const next = new URL(location, currentUrl);
+    if (next.origin !== origin) {
+      throw new JmapIndexError(
+        `JMAP session redirected off-origin (${next.origin}); refusing to send credentials there`,
+      );
+    }
+    currentUrl = next.toString();
+  }
+
+  if (!response) throw new JmapIndexError('JMAP session fetch produced no response');
   if (response.status === 401 || response.status === 403) {
     throw new JmapIndexError('JMAP authentication failed', 401);
   }
+  if (response.status >= 300 && response.status < 400) {
+    throw new JmapIndexError('Too many redirects fetching the JMAP session');
+  }
   if (!response.ok) {
     throw new JmapIndexError(`JMAP session fetch failed (${response.status})`);
   }