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.

.dockerignore

@@ -9,3 +9,8 @@ scripts/
 TODO.md
 *.md
 !README.md
+# Sibling projects / test harness - not part of the webmail image
+examples/
+integration/
+e2e/
+**/node_modules