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/comfyui/download-http-lib.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# Shared HTTP download helpers for ComfyUI model scripts.
+# shellcheck disable=SC2034
+download_http_file() {
+  local url="$1"
+  local dest="$2"
+  local data_root="${DATA_ROOT:-/data}"
+  local dest_dir
+  dest_dir="$(dirname "${dest}")"
+  echo "Downloading ${url} ..."
+  echo "Target: ${dest}"
+
+  if [[ -w "${dest_dir}" ]]; then
+    if [[ -n "${HF_TOKEN:-}" ]]; then
+      if command -v wget >/dev/null 2>&1; then
+        wget -c --progress=dot:giga \
+          --header="Authorization: Bearer ${HF_TOKEN}" \
+          -O "${dest}" "${url}"
+      else
+        curl -fL -C - \
+          -H "Authorization: Bearer ${HF_TOKEN}" \
+          -o "${dest}" "${url}"
+      fi
+    else
+      if command -v wget >/dev/null 2>&1; then
+        wget -c --progress=dot:giga -O "${dest}" "${url}"
+      else
+        curl -fL -C - -o "${dest}" "${url}"
+      fi
+    fi
+    return
+  fi
+
+  if docker ps --format '{{.Names}}' 2>/dev/null | grep -qx 'comfyui'; then
+    local container_dest="/root/ComfyUI/models${dest#"${data_root}/apps/comfyui/models"}"
+    echo "Host directory not writable — using comfyui container: ${container_dest}"
+    if [[ -n "${HF_TOKEN:-}" ]]; then
+      docker exec comfyui wget -c --progress=dot:giga \
+        --header="Authorization: Bearer ${HF_TOKEN}" \
+        -O "${container_dest}" "${url}"
+    else
+      docker exec comfyui wget -c --progress=dot:giga \
+        -O "${container_dest}" "${url}"
+    fi
+    return
+  fi
+
+  echo "ERROR: Cannot write to ${dest_dir}"
+  echo "Fix: sudo chown -R \$USER:\$USER ${data_root}/apps/comfyui/models"
+  echo "Or start ComfyUI: cd stacks/comfyui && ./scripts/start.sh"
+  exit 1
+}