ci: Playwright e2e of the full in-browser cycle against a real server

[JArmandoAnaya]

Context

The browser-level proof of the milestone: the whole cycle, real server, real kernel, no
mocks.

Deliverables

• CI job: build app → start visionset ui against a tmp workspace → Playwright drives
  token entry → project+schema → ingest generated video → approve → annotate 3 assets
  (bbox+polygon+tag) → complete → promote → publish → export (dummy) → download.
• Stable selectors policy (data-testid) documented for ui-core components.

Acceptance criteria

• e2e green in CI, runtime under ~5 min
• Flake policy: retries + trace artifacts uploaded on failure

[JArmandoAnaya]

Verification pass (2026-07-31): the Playwright harness this issue assumed it would create now exists — #48 (M4) stood it up: frontend/app/playwright.config.ts, specs under frontend/app/e2e/, a CI job with browser caching (.github/workflows/ci.yml), and the data-testid selector policy already exercised against the demo page. This issue therefore extends that harness with the full-cycle spec against a real visionset ui server (real workspace, no mocks) rather than standing up a second one — one config, one CI job family, additive specs. The deliverable about documenting the selector policy is partially discharged; keep #48's conventions.