feat: add pos system backup — encrypted folder snapshots with --service picker

bin/pos

@@ -40,7 +40,7 @@ CATEGORIES
   network    ip | checkport | scan | hotspot
   docker     ps | compose
   media      mp3 | mp4
-  system     firewall
+  system     firewall | backup
   ssh        load-keys
   vbox       create | enter | stop | start | rm | ls
 
@@ -58,6 +58,8 @@ EXAMPLES
   pos media mp4 <url>                     Download video as MP4
 
   pos system firewall                     Interactive UFW manager
+  pos system backup /srv/project          Encrypted (AES-256) folder snapshot
+  pos system backup --service             Pick a folder from /srv or ~/srv
 
   pos ssh load-keys                       Load all SSH keys into agent
 
@@ -110,7 +112,7 @@ MAIN_LOG="$LOG_DIR/pos.log"
 log_cmd() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $* → exit $2" >> "$MAIN_LOG"; }
 
 # Commands that read from stdin interactively — only log invocation
-INTERACTIVE_CMDS="system-firewall media-mp4"
+INTERACTIVE_CMDS="system-firewall media-mp4 system-backup"
 
 for ((i=n-1; i>=0; i--)); do
     cmd="pos"