Comprehensive code review fixes: memory safety, resource management, and GTK4 modernization

[Copilot]

This PR addresses numerous critical issues identified during a comprehensive code review of the victus-control application, focusing on memory safety, resource management, input validation, and modern GTK4 API usage.

Backend Critical Fixes

Memory Safety & Resource Management

• Fixed file descriptor leaks: Added explicit close() calls for all file streams in fan.cpp and keyboard.cpp
• Added missing headers: Included errno, cstring, and signal headers to prevent undefined behavior
• Fixed socket cleanup: Added proper socket file cleanup and signal handlers for graceful shutdown
• Bounds checking: Added array bounds validation for fan index access to prevent buffer overflows

Input Validation & Security

• Enhanced command validation: Added comprehensive parameter validation for all backend commands:
  • Fan numbers must be 1 or 2
  • Fan speeds must be non-negative integers
  • RGB values must be in range 0-255
  • Brightness values must be in range 0-255
• Command length validation: Added zero-length and maximum length checks for socket commands
• Improved error messages: Enhanced error reporting with detailed exit codes and descriptive messages

Thread Safety

• Fixed race conditions: Resolved synchronization issues in the better_auto thread handling
• Improved thread cleanup: Fixed potential deadlocks in thread shutdown sequences
• Better resource locking: Enhanced mutex usage for shared resources

Frontend Modernization

GTK4 API Updates

• Replaced deprecated APIs:
  • GtkComboBoxText → GtkDropDown with GtkStringList
  • GtkColorChooserWidget → GtkColorButton
  • GtkMessageDialog → Custom dialog with modern components
• Updated signal handlers: Modernized callback signatures and event handling
• Improved UI responsiveness: Enhanced async operations to prevent UI blocking

Enhanced Error Handling

• Better error dialogs: Replaced deprecated message dialogs with modern GTK4 components
• Improved user feedback: Added more descriptive error messages and validation feedback
• Graceful degradation: Better handling of backend connection failures

Technical Improvements

Socket Communication

• Connection validation: Added proper socket state checks and error recovery
• Protocol robustness: Enhanced message validation and error handling
• Resource cleanup: Proper socket closure and file descriptor management

Code Quality

• Fixed compiler warnings: Resolved all critical compilation warnings
• Improved const-correctness: Better use of const qualifiers where appropriate
• Enhanced error propagation: Comprehensive error handling throughout the application stack

Testing

• ✅ All changes compile successfully with GCC 13.3.0
• ✅ Backend builds without errors or critical warnings
• ✅ Frontend builds with only minor deprecation warnings (using GTK4 compatibility layer)
• ✅ Socket communication protocol validated
• ✅ Thread safety improvements verified

Impact

These changes significantly improve the application's stability, security, and maintainability. The fixes address potential crashes, memory leaks, and security vulnerabilities while modernizing the codebase to follow current best practices for C++ and GTK4 development.

The application should now be much more robust in handling edge cases, malformed inputs, and system resource constraints.

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