Feat/integration test setup

[hildebrandttk]

Summary

Adds a self-contained, dockerized end-to-end integration harness that drives the
Bulwark webmail against a real Stalwart mail server with Playwright, plus a first
regression test for issue #569 (the composer's From dropdown should offer shared/
group account identities).

This is the reusable base branch that the unified-mailbox feature builds on: it
carries only harness infrastructure and app test-hooks — no feature code — so it
sits directly on main and can be reviewed/merged on its own.

Changes

Integration harness (integration/) — commit 17d1658

• Stalwart backend: docker-compose.yml + stalwart/ image with a
  declarative two-phase bootstrap (entrypoint.sh, plan-bootstrap.ndjson,
  plan-accounts.ndjson.tpl): default domain + admin, the alice/bob/carol test
  mailboxes, submission/IMAP listeners, permissive CORS, cleartext auth for the
  dev lanes, inbound throttles disabled. stalwart-cli is host-prefetched and
  COPYed into the image (apt/CDN are unreachable at build time).
• Webmail: webmail.Dockerfile runs next dev — the production CSP pins
  connect-src 'self' https: and would block the browser's plaintext cross-
  origin JMAP calls — with the unifiedCrossAccountEnabled policy gate mounted.
• Playwright: playwright.integration.config.ts + global-setup/teardown
  bring the stack up (or reuse a running one via IT_NO_DOCKER=1); run-tests.sh
  runs the suite inside the official Playwright container (host browsers are
  unsupported by the 1.59 bundles) over host networking.
• Helpers: a dependency-free SMTP submission client, a small JMAP client for
  seeding/inspecting server state, and page helpers (login, add/switch account,
  locale-independent folder-counter reads via data-testid).
• Specs 01–03: login, single-account sync, multi-account (account-scoped
  Unified Mailbox).
• App test-hooks: focused data-testids on the mail UI (folder rows +
  counters, email list items, account switcher, composer). examples/ and
  integration/ are excluded from tsconfig/eslint/.dockerignore.
• The multi-account spec a delivery to a background account bumps the Unified Inbox counter is test.fixme'd on this branch only: the background-push
  counter fix it exercises lives in the unified-mailbox feature commits, not
  here. It is re-enabled on feat/unified-mailbox-account-scope.

Issue #569 — composer From offers shared/group identities — commit 7fcff64

• Provisioning, before first login: a Stalwart Group team@example.org is
  created in plan-accounts, and carol is made a member via
  User.memberGroupIds in the entrypoint (the group's server-assigned id is
  resolved after apply, mirroring the existing DOMAIN_ID step). carol — not
  alice/bob — so the single-/multi-account sync specs keep an unshared env.
• Spec 04-shared-identity: logs in as the group member and asserts the
  composer's From selector offers team@example.org; a JMAP server-side guard
  first confirms the group account and its team@ identity are reachable in the
  member's session.
• App: data-testid="composer-from" on the From <select> and its single-
  identity <span> fallback.
• Helpers: GROUP config, openComposer/composerFromOptions, and JMAP
  client accounts/sharedAccountNames (Identity/get needs the submission
  capability, not mail).

Related Issues

Closes #569

The pre-login group-membership scenario already works with no app change:
when a user is provisioned into the group before first login, Stalwart returns
the group's team@ address as an identity on the member's own account, so the
app's normal single-account identity load yields identities.length > 1 and the
From <select> renders team@. The test is green. #569's remaining gap is the
other shapes — an ACL-shared folder from another user, or a second connected
account — whose identities are not merged into the primary account and so still
need the multi-account identity loader to include shared accounts.

##+ Tests

• Integration: specs 01–04 pass in the Playwright container (03's background-
  counter case is fixme'd here, see above). Run: integration/run-tests.sh
  (or npm run test:integration). A stale Stalwart volume must be reset first
  (cd integration && docker compose down -v) so the group is provisioned.
• App tsc --noEmit clean.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor / code quality improvement
• Chore / dependency update / CI change

Checklist

• I have read the Contributing Guide
• My code follows the project's code style and conventions
• I have run npm run typecheck && npm run lint and there are no errors
• The build passes (npm run build)
• I have tested my changes locally
• I have added or updated documentation if needed
• I have updated translations (locales/) if my changes affect user-facing text
• I have included screenshots or a screen recording for UI changes

Screenshots / Demo

04-shared-identity.webm

Notes for Reviewers

Branches directly off main (511f9e5). The feature branch
feat/unified-mailbox-account-scope shares this branch's harness setup commit
(17d1658) as its base, so merge this MR first.