test(integration): dockerized webmail⇆Stalwart Playwright sync suite

Add an end-to-end integration harness that runs the webmail against a real
Stalwart mail server in Docker and drives it with Playwright, focused on the
mail/folder synchronisation behaviour (unread/total counters, folder-list
sync, account-scoped Unified Mailbox) that the unified-mailbox work touches.

- integration/ stack: Stalwart (declarative bootstrap: alice/bob/carol,
  submission + IMAP listeners, permissive CORS) + webmail (dev mode, so the
  browser's plaintext cross-origin JMAP calls aren't blocked by the prod CSP).
- Helpers: dependency-free SMTP submit client, JMAP client for seeding /
  inspecting server state, and page helpers (login, add/switch account,
  locale-independent folder-counter reads).
- Specs: login, single-account sync (receive/read/move/delete/folder-create/
  burst) and multi-account (per-account isolation + cross-account Unified
  Inbox aggregation + background-account delivery). 12 tests, all green.
- Add focused data-testid hooks to the mail UI (folder rows + counters, email
  list items, account switcher, composer) for stable selectors.
- Exclude examples/ and integration/ from tsconfig/eslint/.dockerignore.

integration/.env.example

@@ -0,0 +1,11 @@
+# Credentials for the integration-test Stalwart mail server.
+# Copy to `.env` (docker compose reads it automatically): cp .env.example .env
+
+# Recovery admin (format user:password). Stays valid after bootstrap so the
+# Stalwart admin UI on http://localhost:8025 remains reachable and stalwart-cli
+# can be invoked via `docker exec`.
+STALWART_RECOVERY_ADMIN=admin:bootstrap-secret
+
+# Shared password for every test mailbox (alice/bob/carol @ example.org).
+# The Playwright harness reads the same value from IT_ACCOUNT_PASSWORD.
+TEST_ACCOUNT_PASSWORD=test-pass-123