Fix Plesk helper sourcing and add fallback for domain discovery

Problem: When domain-discovery.sh is sourced directly (not via launcher),
plesk-helpers.sh wasn't being loaded because $LIB_DIR was undefined.
This caused list_all_domains() to fail on Plesk with 'command not found'.

Fixes:
1. Enhanced Plesk helper sourcing logic:
   - Try $LIB_DIR first (when sourced from launcher)
   - Fall back to $SCRIPT_DIR (when sourced directly)
   - Ensures plesk-helpers.sh loads in all contexts

2. Added fallback in list_all_domains() for Plesk:
   - Check if plesk_list_domains function exists
   - If not available, fall back to directory scan
   - Scans /var/www/vhosts/ excluding system directories
   - Ensures domains are found even without plesk-helpers.sh

Impact: Domain discovery now works correctly when:
- Sourced from launcher (uses plesk-helpers.sh)
- Sourced directly from command line (uses fallback)
- Plesk CLI unavailable (uses directory scan)

Tested on: Plesk 18.0.61 production system

lib/domain-discovery.sh

@@ -13,7 +13,14 @@ if [ -z "$TOOLKIT_BASE_DIR" ]; then
 fi
 
 # Source control panel helpers if available
-[ "$SYS_CONTROL_PANEL" = "plesk" ] && [ -f "$LIB_DIR/plesk-helpers.sh" ] && source "$LIB_DIR/plesk-helpers.sh"
+if [ "$SYS_CONTROL_PANEL" = "plesk" ]; then
+    # Try LIB_DIR first, then SCRIPT_DIR
+    if [ -n "$LIB_DIR" ] && [ -f "$LIB_DIR/plesk-helpers.sh" ]; then
+        source "$LIB_DIR/plesk-helpers.sh"
+    elif [ -n "$SCRIPT_DIR" ] && [ -f "$SCRIPT_DIR/plesk-helpers.sh" ]; then
+        source "$SCRIPT_DIR/plesk-helpers.sh"
+    fi
+fi
 
 #############################################################################
 # DOMAIN DISCOVERY (Control Panel Agnostic)
@@ -36,7 +43,15 @@ list_all_domains() {
             ;;
 
         plesk)
+            # Use plesk_list_domains if available, otherwise fallback
+            if type plesk_list_domains >/dev/null 2>&1; then
                 plesk_list_domains
+            else
+                # Fallback: scan vhosts directory
+                ls -1 /var/www/vhosts/ 2>/dev/null | \
+                    grep -v "^system$\|^chroot$\|^\.skel$\|^default$\|^fs$" | \
+                    grep -v "^\." || true
+            fi
             ;;
 
         interworx)