fix(backup): read remote backup list from stdin in selector

select_newest_remote_backup_from_list was sorting an empty process
substitution instead of the piped list-backups.sh output.

Co-authored-by: Cursor <cursoragent@cursor.com>

volume-utils.sh

@@ -66,9 +66,20 @@ select_newest_remote_backup_from_list() {
   local remote_source=$1
   local volume_name=$2
   local newest="" f size
+  local -a files=()
 
   while IFS= read -r f; do
-    [[ -z "$f" ]] && continue
+    [[ -n "$f" ]] && files+=("$f")
+  done
+
+  if (( ${#files[@]} == 0 )); then
+    return 0
+  fi
+
+  IFS=$'\n' files=( $(printf '%s\n' "${files[@]}" | sort) )
+  unset IFS
+
+  for f in "${files[@]}"; do
     [[ "$f" == "${volume_name}-"* ]] || continue
     [[ "$f" == *.tar.zst ]] || continue
     size=$(remote_backup_archive_size_bytes "$remote_source" "$f")
@@ -77,7 +88,7 @@ select_newest_remote_backup_from_list() {
       continue
     fi
     newest="$f"
-  done < <(sort)
+  done
 
   if [[ -n "$newest" ]]; then
     echo "$newest"