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Comprehensive fix: security, bugs, refactoring, tests, and rsync parity features
fix: resolve all security issues (#154, #156, #157, #159, #160, #161, #162, #170)
fix: resolve all security issues (#154, #156, #157, #159, #160, #161, #162, #170)
Batch fix: NULL checks, input validation, and code refactoring (#152, #149, #150, #151)
refactor: config_create(), main(), send_files() — reduce duplication and complexity (#149, #150, #151)
Batch fix: NULL checks, input validation, and code refactoring (#152, #149, #150, #151)