test(electron): smoke test as the regression gate for the desktop shell

Phase 1 step 2 of VNCprodbuild. e2e/electron-smoke.spec.ts uses Playwright's
_electron.launch() to boot the real skeleton (dist-electron/main.js from
step 1) and asserts:
  - the login screen renders (same input[type="text"]/[type="password"]
    selectors as e2e/login.spec.ts's browser-based check)
  - zero uncaught page errors fire during load

Sets JMAP_SERVER_URL (any non-empty value) so the app reaches
lib/setup/state.ts's "env-managed" state and serves the normal login screen
instead of 302ing to the first-run /setup wizard - no live mail server or
mock JMAP build flag needed just to prove the shell renders.

playwright.electron.config.ts is deliberately separate from
playwright.config.ts: it has no `webServer` block, since this suite's app
boots its own server and would otherwise race pointlessly with `npm run dev`
starting on :3000 for the browser-based e2e/*.spec.ts suite.

Wired as `npm run test:electron`. Verified green locally (2 passed) after
`npm run build:standalone && npm run build:electron`; every later step in
the Electron rollout must keep this passing before moving on.

.gitignore

@@ -42,6 +42,10 @@ yarn-error.log*
 /dist-electron/
 /dist-electron-builds/
 
+# playwright output
+/test-results/
+/playwright-report/
+
 # typescript
 *.tsbuildinfo
 next-env.d.ts