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/css/components.css

@@ -313,6 +313,21 @@ input:-webkit-autofill:focus {
     color: white;
 }
 
+.overlay-preview-img {
+    max-width: 80vw;
+    max-height: 50vh;
+    border-radius: 8px;
+    margin-top: 16px;
+    object-fit: contain;
+}
+
+.overlay-preview-stats {
+    color: rgba(255, 255, 255, 0.7);
+    font-size: 0.85rem;
+    margin-top: 8px;
+    font-variant-numeric: tabular-nums;
+}
+
 .toast {
     position: fixed;
     bottom: 40px;