feat: replace Python seam carving with caire Go binary

Replace the Python seam-carving library with caire (esimov/caire v1.5.0),
a Go-based content-aware resize engine that is faster and supports both
shrinking and enlarging via seam insertion.

- Add Go builder stage in Dockerfile to compile caire from source
- Rewrite seam-carving.ts to call caire via execFile (no Python sidecar)
- Remove content-aware-resize from PYTHON_SIDECAR_TOOLS (60s timeout)
- Add new options: blur radius, edge sensitivity, square mode, face detection
- Move content-aware toggle below standard resize in UI (subtler placement)
- Rename "Don't enlarge" to "Limit to original size" with hover tooltip
- Add smooth progress bar for medium-duration tools
- Delete seam_carve.py and remove seam-carving pip dependency
- Update integration tests and visual regression screenshots

docker/download_models.py

@@ -110,9 +110,8 @@ def smoke_test():
     from PIL import Image
     import cv2
     import numpy
-    import seam_carving
     from rembg import new_session
-    print("  CPU imports OK (Pillow, cv2, numpy, seam_carving, rembg)")
+    print("  CPU imports OK (Pillow, cv2, numpy, rembg)")
 
     # MediaPipe is CPU-only, should always import
     import mediapipe as mp