[Linux] Implement KeyboardMonitor class using X11/XInput2

[Copilot]

This PR implements the missing KeyboardMonitor class for Linux platforms, completing the cross-platform keyboard monitoring functionality.

Overview

The Linux implementation uses X11 with the XInput2 extension to provide global keyboard event monitoring, following the same architectural patterns as the existing Windows and macOS implementations.

Key Features

• Global keyboard monitoring: Captures key press/release events system-wide using XInput2
• Modifier key detection: Tracks changes to Shift, Ctrl, Alt, Meta, Caps Lock, Num Lock, and Scroll Lock
• Thread-safe operation: Background monitoring thread prevents blocking the main application
• Graceful error handling: Properly handles cases where X11 display is unavailable (headless environments)
• Resource management: Automatic cleanup of X11 resources on stop/destruction

Technical Implementation

The implementation follows the established PIMPL pattern used by other platforms:

class KeyboardMonitor::Impl {
  Display* display_;
  std::atomic<bool> monitoring_;
  std::thread monitoring_thread_;
  // X11/XInput2 event handling
};

Key aspects:

• Uses XISelectEvents to register for keyboard events on the root window
• Processes XI_KeyPress and XI_KeyRelease events in a dedicated monitoring thread
• Converts X11 keycodes to the common event handler interface
• Resolves naming conflicts between X11's None macro and the ModifierKey::None enum

Dependencies

Adds the following Linux-specific dependencies to the build system:

• libx11-dev - Core X11 library
• libxi-dev - XInput2 extension for input device handling
• pthread - Threading support

Testing

The implementation has been validated to:

• Build successfully on Linux with GTK3/X11 dependencies
• Properly integrate with the existing cross-platform CMake build system
• Handle start/stop operations correctly
• Manage monitoring state appropriately
• Gracefully handle missing X11 display connections

This completes the keyboard monitoring functionality across all supported platforms (Windows, macOS, and Linux).

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