feat: upgrade to BiRefNet SOTA background removal model

- Switch default model from U2-Net to BiRefNet (state-of-the-art)
- Add 6 model options: BiRefNet, BiRefNet Lite, BiRefNet Portrait,
  BRIA RMBG, IS-Net, U2-Net
- Add animated progress bar with stage indicators (loading model,
  analyzing, removing, refining edges) and elapsed timer
- Add intuitive background color presets (Transparent, White, Black,
  Red, Green, Blue) as clickable buttons + custom color picker
- Handle background color compositing in Python (PIL alpha composite)
- Add checkerboard pattern to before/after slider for transparency
- Pre-bake BiRefNet model (973MB) in Docker image for instant use

docker/Dockerfile

@@ -71,10 +71,12 @@ RUN /opt/venv/bin/pip install --no-cache-dir --upgrade pip && \
 # This makes the Docker image fully self-contained — works offline
 RUN /opt/venv/bin/python3 -c "\
 from rembg import new_session; \
-print('Downloading u2net model...'); \
+print('Downloading BiRefNet model (SOTA)...'); \
+new_session('birefnet-general'); \
+print('Downloading u2net model (fallback)...'); \
 new_session('u2net'); \
-print('u2net model ready') \
-" 2>/dev/null || echo "WARNING: Could not pre-download u2net model"
+print('Background removal models ready') \
+" 2>/dev/null || echo "WARNING: Could not pre-download rembg models"
 
 RUN /opt/venv/bin/python3 -c "\
 try: \