Add stack Update, ComfyUI model manager, and slim ComfyUI stack.

Server UI gains Update on stack cards, ComfyUI Models tab with workflow
scan and downloads, and centralized comfyui_config. Model catalog and
download scripts move from stacks/comfyui to server-ui so ComfyUI stays a
minimal Docker wrapper for easier image updates.

stacks/server-ui/scripts/install.sh

@@ -49,6 +49,8 @@ rsync -a --delete \
   --exclude '__pycache__' \
   "${STACK_DIR}/" "${INSTALL_DIR}/"
 
+install -m 644 "$(dirname "${STACK_DIR}")/control-plane/env_loader.py" "${INSTALL_DIR}/env_loader.py"
+
 python3 -m venv "${INSTALL_DIR}/.venv"
 "${INSTALL_DIR}/.venv/bin/pip" install --upgrade pip -q
 "${INSTALL_DIR}/.venv/bin/pip" install -r "${INSTALL_DIR}/requirements.txt" -q