Add live preview streaming for capture tests via WebSocket

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>

server/src/wled_controller/static/locales/en.json

@@ -903,6 +903,9 @@
   "value_source.test.current": "Current",
   "value_source.test.min": "Min",
   "value_source.test.max": "Max",
+  "test.frames": "Frames",
+  "test.fps": "FPS",
+  "test.avg_capture": "Avg",
   "targets.brightness_vs": "Brightness Source:",
   "targets.brightness_vs.hint": "Optional value source that dynamically controls brightness each frame (overrides device brightness)",
   "targets.brightness_vs.none": "None (device brightness)",