Fix test endpoints reporting pre-filter image dimensions

- WebSocket test: move w,h capture to after PP filter application
  so downscaler effect is reflected in reported resolution
- HTTP test: read actual thumbnail dimensions from filtered image
  instead of using pre-computed values

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/routes/postprocessing.py

@@ -298,6 +298,7 @@ async def test_pp_template(
         actual_fps = frame_count / actual_duration if actual_duration > 0 else 0
         avg_capture_time_ms = (total_capture_time / frame_count * 1000) if frame_count > 0 else 0
         width, height = pil_image.size
+        thumb_w, thumb_h = thumbnail.size
 
         return TemplateTestResponse(
             full_capture=CaptureImage(
@@ -305,8 +306,8 @@ async def test_pp_template(
                 full_image=full_data_uri,
                 width=width,
                 height=height,
-                thumbnail_width=thumbnail_width,
-                thumbnail_height=thumbnail_height,
+                thumbnail_width=thumb_w,
+                thumbnail_height=thumb_h,
             ),
             border_extraction=None,
             performance=PerformanceMetrics(