Fix SOURCE command safety issues (HIGH priority)

Added existence checks and error handling for all source commands
to prevent silent failures when dependencies are missing.

Library files (use 'return' for error):
- reference-db.sh: Added checks for 3 dependencies
- mysql-analyzer.sh: Added checks for 3 dependencies
- domain-discovery.sh: Added checks for 2 dependencies
- system-detect.sh: Added check for common-functions.sh
- plesk-helpers.sh: Added check for common-functions.sh
- user-manager.sh: Added checks for 2 dependencies

Executable scripts (use 'exit' for error):
- wordpress-cron-manager.sh: Added checks for 2 dependencies
- website-error-analyzer.sh: Added checks for 4 dependencies

Pattern: [ -f "file" ] && source "file" || { echo "ERROR" >&2; return/exit 1; }

This ensures scripts fail fast with clear error messages when
required dependencies are missing, rather than continuing with
undefined functions.

lib/plesk-helpers.sh

@@ -8,7 +8,7 @@
 # Source common functions if not already loaded
 if [ -z "$TOOLKIT_BASE_DIR" ]; then
     SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-    source "$SCRIPT_DIR/common-functions.sh"
+    [ -f "$SCRIPT_DIR/common-functions.sh" ] && source "$SCRIPT_DIR/common-functions.sh" || { echo "ERROR: common-functions.sh not found" >&2; return 1; }
 fi
 
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