Replace blocking REST-based capture tests with WebSocket endpoints that
stream intermediate frame thumbnails at ~100ms intervals, giving real-time
visual feedback during capture. Preview resolution adapts dynamically to
the client viewport size and device pixel ratio.
- New shared helper (_test_helpers.py) with engine_factory pattern to
avoid MSS thread-affinity issues
- WS endpoints for stream, capture template, and PP template tests
- Enhanced overlay spinner with live preview image and stats
- Frontend _runTestViaWS shared helper replaces three REST test runners
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
alexei.dolgolyov
b8bfdac36b