CRITICAL FIX: plesk-helpers.sh was never loaded - wrong path

Issue: system-detect.sh tried to source $SCRIPT_DIR/plesk-helpers.sh
but plesk-helpers.sh is in lib/ directory.

Fix: Changed to ${LIB_DIR:-$SCRIPT_DIR/lib}/plesk-helpers.sh

This caused ALL Plesk helper functions to be unavailable:
- plesk_list_domains()
- plesk_get_owner()
- plesk_get_docroot()
- etc.

Result: Plesk servers showed 0 users, 0 domains, 0 databases

Testing: Will verify on Plesk server after push

lib/system-detect.sh

@@ -68,8 +68,8 @@ detect_control_panel() {
         SYS_LOG_DIR="/var/www/vhosts/system"
 
         # Source Plesk helpers for advanced functionality
-        if [ -f "$SCRIPT_DIR/plesk-helpers.sh" ]; then
+        if [ -f "${LIB_DIR:-$SCRIPT_DIR/lib}/plesk-helpers.sh" ]; then
-            source "$SCRIPT_DIR/plesk-helpers.sh"
+            source "${LIB_DIR:-$SCRIPT_DIR/lib}/plesk-helpers.sh"
         fi
 
         print_success "Detected Plesk v${SYS_CONTROL_PANEL_VERSION}"