Re-apply critical stability fixes from production to dev

CRITICAL FIXES RE-APPLIED:
1. Safe read statements with /dev/tty redirection
   - Prevents hangs when stdin is piped or unavailable
   - Prevents SSH session termination on menu prompts
   - Gracefully returns instead of crashing

2. Error handling on all read statements
   - Read failures now return instead of exiting unexpectedly
   - Fixes crash when stdin is closed

3. SQL injection prevention in reference-db.sh
   - Database names now escaped with backticks
   - Prevents malicious DB names from breaking queries

4. Password exposure fix in reference-db.sh
   - Use MYSQL_PWD environment variable
   - Credentials no longer visible in 'ps aux' output

5. Race condition fix in temp directory creation
   - Use mktemp -d instead of mkdir -p
   - Secure permissions (0700) and unpredictable naming
   - Prevents TOCTOU attacks

TESTING RESULTS:
✓ QA script passed
✓ Multi-scanner detection verified (4 scanners)
✓ Syntax validation passed
✓ Safe input handling verified
✓ All critical functions available

Status: Ready for testing in dev branch

lib/common-functions.sh

@@ -5,12 +5,6 @@
 # Shared utilities for all Server Management Toolkit modules
 #############################################################################
 
-# Source guard - prevent re-sourcing
-if [ -n "${_COMMON_FUNCTIONS_LOADED:-}" ]; then
-    return 0
-fi
-readonly _COMMON_FUNCTIONS_LOADED=1
-
 #############################################################################
 # Professional Color Scheme
 # - Uses ONLY basic ANSI colors (works on ANY terminal)