Harden step_snap: dedupe Set ids (multi-snap sets)

Local flow verified end-to-end (macOS guards). Remote Ubuntu/Arch
verification deferred to user (interactive, destructive on real hosts).

Co-Authored-By: Claude <noreply@anthropic.com>

cleanup-server

@@ -278,7 +278,8 @@ step_snap() {
         return
     fi
     local saved
-    saved=$(sudo snap saved 2>/dev/null | awk 'NR>1{print $1}')
+    # Cột đầu (Set) là id cần forget; một Set có thể có nhiều Snap → dedupe.
+    saved=$(sudo snap saved 2>/dev/null | awk 'NR>1{print $1}' | awk '!seen[$0]++')
     if [ -z "$saved" ]; then
         echo "${C_OK}    ✓ Không có snap snapshot nào.${C_RESET}"
         echo ""