perf: Implement Nth frame processing for webcam mode

Optimizes webcam performance by running full face detection and
frame processing (face swap, enhancers) only every N frames
(currently N=3) in modules/ui.py (create_webcam_preview function).

For intermediate frames, the raw (but mirrored/resized) webcam
feed is displayed. This aims to improve UI responsiveness and reduce
overall CPU/GPU load during live webcam sessions, particularly when
resource-intensive operations like hair swapping or face enhancement
are active.

The actual swap/effect will appear at a reduced frame rate (FPS/N),
but the UI should remain smoother.

modules/ui.py

@@ -988,6 +988,9 @@ def create_webcam_preview(camera_index: int):
 
     # --- End Source Image Loading ---
 
+    detection_frame_counter = 0
+    DETECTION_INTERVAL = 3 # Process every 3rd frame
+
     prev_time = time.time()
     fps_update_interval = 0.5
     frame_count = 0
@@ -1013,6 +1016,8 @@ def create_webcam_preview(camera_index: int):
                 temp_frame, PREVIEW.winfo_width(), PREVIEW.winfo_height()
             )
 
+        detection_frame_counter += 1
+        if detection_frame_counter % DETECTION_INTERVAL == 0:
             if not modules.globals.map_faces:
                 # Case 1: map_faces is False - source_face_obj_for_cam and source_frame_full_for_cam are pre-loaded
                 if source_face_obj_for_cam is not None and source_frame_full_for_cam is not None: # Check if valid after pre-loading
@@ -1022,9 +1027,7 @@ def create_webcam_preview(camera_index: int):
                                 temp_frame = frame_processor.process_frame(None, temp_frame)
                         else:
                             temp_frame = frame_processor.process_frame(source_face_obj_for_cam, source_frame_full_for_cam, temp_frame)
-            # If source image was invalid (e.g. no face), source_face_obj_for_cam might be None.
+                # If source image was invalid, processors are skipped; temp_frame remains raw (but mirrored/resized).
-            # In this case, the frame processors that need it will be skipped, effectively just showing the raw webcam frame.
-            # The error message is already persistent due to the pre-loop check.
             else:
                 # Case 2: map_faces is True - source_frame_full_for_cam_map_faces is pre-loaded
                 if source_frame_full_for_cam_map_faces is not None: # Check if valid after pre-loading
@@ -1032,12 +1035,11 @@ def create_webcam_preview(camera_index: int):
                     for frame_processor in frame_processors:
                         if frame_processor.NAME == "DLC.FACE-ENHANCER":
                             if modules.globals.fp_ui["face_enhancer"]:
-                            # Corrected: face_enhancer.process_frame_v2 is expected to take only temp_frame
                                 temp_frame = frame_processor.process_frame_v2(temp_frame)
                         else:
-                        # This is for other processors when map_faces is True
                             temp_frame = frame_processor.process_frame_v2(source_frame_full_for_cam_map_faces, temp_frame)
-            # If source_frame_full_for_cam_map_faces was invalid, error is persistent from pre-loop check.
+                # If source_frame_full_for_cam_map_faces was invalid, processors are skipped.
+        # On non-detection frames, temp_frame (already mirrored/resized) is used directly.
 
         # Calculate and display FPS
         current_time = time.time()