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/stalwart/Dockerfile

@@ -0,0 +1,29 @@
+# Stalwart Mail Server with a declarative bootstrap for webmail integration
+# testing.
+#
+# Extends the official image with stalwart-cli and an entrypoint that, on first
+# container start, applies the bootstrap + account plans against the freshly
+# started server. On subsequent starts the `.bootstrap-applied` marker short-
+# circuits both phases and Stalwart boots straight into normal mode.
+#
+# NOTE: stalwart-cli is COPYed in rather than downloaded during the build. The
+# binary is fetched by ./prepare-stalwart-cli.sh (run for you by the Playwright
+# global-setup / integration runner). This keeps the build offline-friendly and
+# avoids the base image's apt sources, which are unreachable in sandboxed CI.
+
+FROM stalwartlabs/stalwart:v0.16
+
+USER root
+
+# Host-prefetched stalwart-cli matching the build architecture.
+COPY stalwart-cli /usr/local/bin/stalwart-cli
+
+RUN mkdir -p /etc/stalwart-bootstrap
+COPY plan-bootstrap.ndjson /etc/stalwart-bootstrap/plan-bootstrap.ndjson
+COPY plan-accounts.ndjson.tpl /etc/stalwart-bootstrap/plan-accounts.ndjson.tpl
+COPY entrypoint.sh /usr/local/bin/stalwart-bootstrap-entrypoint.sh
+RUN chmod +x /usr/local/bin/stalwart-cli /usr/local/bin/stalwart-bootstrap-entrypoint.sh
+
+USER stalwart
+
+ENTRYPOINT ["/usr/local/bin/stalwart-bootstrap-entrypoint.sh"]