Add update-vendor.sh symlink support, unify install.sh vendor flow

- update-vendor.sh now creates webui/vendor symlinks (htm.js)
- install.sh calls update-vendor.sh after package install
- Add vendor/.empty and webui/vendor/.empty as directory placeholders in git

install.sh

@@ -182,6 +182,10 @@ apt-get install -y -qq \
     apache2-utils \
     avahi-daemon
 
+# --- 1b. Vendored libraries ---
+log "Downloading vendored libraries..."
+bash "${PROJECT_DIR}/scripts/update-vendor.sh" || err "update-vendor.sh failed"
+
 # --- 2. Setup users ---
 log "WebUI user: $USER_NAME (group: $USER_GROUP)"