Fix safety, correctness, and modern C++ best practices across codebase

[Copilot]

Code review identified memory leaks, undefined behavior, race conditions, a data-corruption bug, and several C++ antipatterns. This PR addresses the highest-impact issues.

Memory safety & RAII

• Manager: Replaced vector<Student*> with vector<unique_ptr<Student>>. Eliminates manual delete loop in destructor and the memory leak in remove() (pointer was erased from the vector without being freed).
• PID: Replaced raw PIDImpl* with unique_ptr<PIDImpl>, constructor uses make_unique, destructor becomes = default.

Bug fixes

• Manager::remove(): Missing return after successful erasure caused "Student not found!" to always print, even on success.
• Student::setCode(): name_.append(3, ' ') permanently corrupted the name_ member. The intent was to pad the local nameCode copy:

  // Before (bug): mutates the member
  name_.append(3, ' ');
  nameCode.resize(3);
  
  // After: pads only the local copy
  nameCode.append(3, ' ');
  nameCode.resize(3);

Thread safety

• TCPServer: volatile bool running_ → std::atomic<bool>. volatile provides no cross-thread visibility or ordering guarantees.
• Singleton: Non-thread-safe null-check pattern with a leaking raw pointer replaced with the Meyers singleton (static Singleton instance as a function-local static), which is guaranteed atomic since C++11.

Undefined behavior

• ChainOfCommand: IHandler was deleted through a base pointer with no virtual destructor. Added virtual ~IHandler() = default.

Style & portability

• pid.h/pid.cpp: Removed using namespace std; replaced reserved-identifier guards (_PID_H_, _PID_SOURCE_) with #pragma once.
• Account.cpp, Student.cpp: time(NULL) → time(nullptr).
• SharingData.cpp: Added missing override on all four virtual overrides.
• TwoSum.cpp, main.cpp: Fixed int/size_t signed-unsigned comparison warnings.
• Facade.cpp: Replaced GCC-only Elvis operator (s1 ?: new X) with standard ternary (s1 ? s1 : new X).

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.