syncthing.sh: List available folders when on Add a folder

syncthing.sh

@@ -222,6 +222,13 @@ else:
 
 add_folder() {
     header "Add Folder"
+    local container
+    container=$(docker ps --format '{{.Names}}' | grep -i syncthing | head -1 || true)
+    if [ -n "$container" ]; then
+        echo "Available folders in /var/syncthing/data/:"
+        docker exec "$container" ls /var/syncthing/data/ 2>/dev/null | sed 's/^/  /' || warn "Could not list data directory."
+        echo ""
+    fi
     read -rp "Folder path on this device (e.g. /var/syncthing/data/books): " FOLDER_PATH
     [ -n "$FOLDER_PATH" ] || die "Path cannot be empty."
     read -rp "Folder label (human-readable name): " FOLDER_LABEL