qa(ui): add Playwright + axe UI/QA coverage

[UberMetroid]

Adds an external-environment Playwright suite under tests/ui/ covering the root application shell and PWA manifest, service-worker registration, PIN authentication (invalid + valid), the Kanban add / move / delete workflow across all three columns, axe accessibility scans of the authenticated root page and the Kanban board, and a Chromium visual baseline of the empty default board. The configuration uses Playwright 1.50.1 and @axe-core/playwright 4.10.2, requires the target URL via STUDIO_URL, and the matching numeric PIN via GRID_TEST_PIN. A new UI/QA workflow ( .github/workflows/qa-ui.yml ) builds the existing Alpine container, starts it with GRID_PIN=2468 on port 4405, waits for the health endpoint, then runs the Playwright suite with the matching environment variables and uploads the HTML report and test results as artifacts. The application source is unchanged; the only non-test edit is an additive entry in .gitignore for the Playwright and Node test artifacts. The Kanban and visual suites reset the default board through /api/tasks before each test to keep the baseline reproducible, and the visual test allows up to a 2% pixel-ratio difference to absorb minor cross-platform font rendering variation. The color-contrast axe rule is currently disabled because the upstream theme system does not yet meet WCAG AA on the .add-task button; the remaining axe rules still scan the root page and the Kanban board.