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/webmail.Dockerfile

@@ -0,0 +1,33 @@
+# Webmail image for integration testing — runs Next.js in DEVELOPMENT mode.
+#
+# Why dev mode rather than the production Dockerfile at the repo root?
+# The browser talks JMAP directly to Stalwart at http://localhost:8025 (plain
+# HTTP, cross-origin). The app's production Content-Security-Policy pins
+# connect-src to `'self' https:`, which would block that plaintext cross-origin
+# fetch. In development mode proxy.ts widens connect-src to `'self' http:
+# https: ws: wss:` — exactly what a local, TLS-less Stalwart needs. Running
+# from source also ships the integration-test data-testid hooks without a
+# production rebuild.
+#
+# Build context is the repo root (see docker-compose.yml `context: ..`), so the
+# root .dockerignore keeps examples/, integration/ and node_modules out.
+
+FROM node:24-alpine
+WORKDIR /app
+
+# Install dependencies first for layer caching.
+COPY package.json package-lock.json ./
+RUN npm ci
+
+# App source (data-testid hooks included).
+COPY . .
+
+ENV NODE_ENV=development
+ENV NEXT_TELEMETRY_DISABLED=1
+ENV PORT=3000
+ENV HOSTNAME=0.0.0.0
+
+EXPOSE 3000
+
+# Bind to 0.0.0.0 so the published port is reachable from the host/browser.
+CMD ["npx", "next", "dev", "-H", "0.0.0.0", "-p", "3000"]