test(electron): real end-to-end push -> native notification, via SMTP

Phase 1 step 7 of VNCprodbuild. integration/tests/11-electron-notification.spec.ts
launches the actual Electron shell, logs in as alice against this repo's
existing docker-compose Stalwart fixture, injects a message over real SMTP
(same helpers/smtp.ts sendMail() 02-mail-sync.spec.ts uses), and asserts a
native notification fires via electron/main.ts's __notificationCallCount
test hook - proving the full real pipeline, not just the synthetic IPC call
step 3's smoke test exercises: SMTP -> Stalwart -> JMAP push
(lib/jmap/client.ts) -> stores/email-store.ts's handleStateChange ->
handleNewEmailNotification -> the page effect -> lib/electron-bridge.ts ->
the contextBridge/IPC bridge -> electron/main.ts's Notification call.

Runs against a `next dev` server (electron/main.ts's new ELECTRON_LOAD_URL
escape hatch), not the standalone build, because this fixture's Stalwart is
deliberately plain HTTP and production's CSP correctly refuses non-TLS
connections - the identical trade-off integration/webmail.Dockerfile already
makes for the browser-based suite. New playwright.integration-electron.config.ts
+ global-setup-electron.ts (brings up only the `stalwart` compose service,
not `webmail`, which this suite never touches and which may not even be
startable on a given host - see its own header comment) keep this fully
separate from the main dockerized integration run, which has no Electron
binary compatible with that container's platform; playwright.integration.config.ts
gets a matching testIgnore so a plain `npm run test:integration` never tries
to sweep this file in. Wired as `npm run test:integration:electron`.

On "the real WebSocket path": confirmed against this fixture's actual
`stalwartlabs/stalwart:v0.16` (same as the sandbox server) that its
/jmap/ws requires the same Authorization header as every other JMAP
endpoint on the handshake itself, which the browser WebSocket API cannot
attach - so the WS attempt reaches the network correctly (see the CSP fix
in the previous commit) but always fails auth here, and the circuit
breaker falls back to SSE within about a second. That fallback is what
delivers the push this test observes - documented in detail in the spec's
header comment, including why asserting the WS handshake itself succeeds
here would be asserting something that cannot be true from a browser
against this specific server.

Known flakiness, root-caused not eliminated (see
playwright.integration-electron.config.ts's retries: 2 and its comment):
`next dev`'s on-demand route compilation + Fast Refresh occasionally races
the SSE stream during the login -> inbox transition and drops that one push
event with no error anywhere - reproduced by running the identical test
repeatedly against an already-warm stack (IT_NO_DOCKER=1): identical
request sequence logged every time, but the outcome wasn't always the same.
This is specific to the dev-server workaround this test needs for the
plaintext-Stalwart fixture, not a bug in the feature it's verifying - the
WS circuit breaker and SSE fallback fire exactly as designed in every run's
own logs, pass or fail.

Verified: passed cleanly standalone multiple times; with retries: 2 in
place, passed within the retry budget on every attempt made.

playwright.integration.config.ts

@@ -24,6 +24,13 @@ const VIDEO: VideoMode = VIDEO_MODES.includes(process.env.IT_VIDEO as VideoMode)
 
 export default defineConfig({
   testDir: './integration/tests',
+  // Electron's own spec runs under playwright.integration-electron.config.ts
+  // instead (see that file's header comment for why): the dockerized run
+  // this config drives (integration/run-tests.sh, inside the official
+  // Playwright image) has no Electron binary compatible with that
+  // container's platform, so it must never be swept in by this config's
+  // default testDir glob.
+  testIgnore: '11-electron-notification.spec.ts',
   // next dev compiles routes lazily and each test logs in fresh, so give
   // individual tests and their polling assertions generous headroom.
   timeout: 90_000,