CI: add Playwright e2e job for dashboard smoke + optional screenshot artifact #215

PR #176 added Playwright e2e coverage (e2e/session-dashboard.spec.ts) and an optional docs screenshot regen path gated by ECHO_CAPTURE_DASHBOARD_SCREENSHOT=1.

Right now .github/workflows/ci.yml runs Rust fmt/clippy/tests/guards only; it does not run Playwright, so:

• e2e regressions won't be caught in PRs
• screenshot drift is only caught manually

Proposal

Add a Playwright job to CI, with a cautious default to avoid flakiness / long runtimes:

Option A (recommended): PR smoke + artifacts

• Run pnpm exec playwright test e2e/session-dashboard.spec.ts on PRs.
• Upload Playwright report + screenshot attachments as artifacts.
• Keep ECHO_CAPTURE_DASHBOARD_SCREENSHOT off in CI (don't mutate repo), but still attach the PNG to the CI artifacts so humans can compare.

Option B: manual / nightly screenshot regen

• Add a workflow_dispatch job that runs the same test with ECHO_CAPTURE_DASHBOARD_SCREENSHOT=1 and uploads the produced docs/assets/wvp/session-dashboard.png as an artifact (or opens a PR via bot if desired).

Implementation notes

• Node is already in-repo (package.json, pnpm-lock.yaml), so CI job should use pnpm with cache.
• Install browsers: pnpm exec playwright install --with-deps on ubuntu runners (or install on macOS).
• Keep timeouts reasonable and avoid parallel test runs if flake observed.

Acceptance criteria

• New CI workflow job runs Playwright e2e (at least e2e/session-dashboard.spec.ts) on PRs.
• Job uploads artifacts (Playwright report and attached screenshot).
• Job is reliable (no persistent flakes) or can be label-gated if needed.