Add web UI E2E tests with Playwright #55
Description
Summary
Add browser-based end-to-end tests for the web UI (ros2_medkit_web_ui) using Playwright, running against the full demo stack in selfpatch_demos.
Motivation
selfpatch_demos is the only place where the full stack is composed (gateway + demo nodes + web UI via docker-compose). The web UI repo itself has no demo infrastructure and cannot run meaningful E2E tests. Tests should verify:
- Entity tree renders and shows expected areas/components/apps
- Clicking an entity shows correct detail panel with data/configurations/faults tabs
- Live data updates (SSE/polling)
- Fault injection reflected in UI
Depends on #54 (pytest-playwright migration) which establishes the shared test infrastructure. Playwright's browser context can be used alongside the API request context from the same conftest.py fixtures.