This PR implements the missing WindowManager class for Linux platform using GTK/GDK APIs, completing the cross-platform window management functionality.

Problem

The repository had WindowManager implementations for macOS and Windows, but Linux was missing, causing build failures and preventing Linux users from utilizing window management features:

// This would fail to link on Linux
WindowManager windowManager;
auto currentWindow = windowManager.GetCurrent();
auto allWindows = windowManager.GetAll();

Solution

Added complete Linux implementation with GTK3 backend:

Core Implementation

• src/platform/linux/window_manager_linux.cpp - Implements the three core methods:

  • Get(WindowID id) - Retrieves window by ID using GTK window enumeration
  • GetAll() - Lists all application windows via gtk_window_list_toplevels()
  • GetCurrent() - Finds focused window using GDK seat/device APIs

• src/platform/linux/window_linux.cpp - Complete Window class implementation with GDK backend supporting all window operations (focus, show/hide, maximize/minimize, positioning, etc.)

Technical Details

• Uses GTK3 APIs for consistency with existing Linux DisplayManager
• Follows PIMPL pattern like macOS implementation but with GdkWindow* instead of NSWindow*
• Uses GdkWindow pointer addresses as unique WindowID values
• Handles headless environments gracefully with gtk_init_check()

Build System Fixes

• Fixed CMake Linux detection (CMAKE_SYSTEM_NAME STREQUAL "Linux" instead of LINUX)
• Corrected Objective-C++ compiler enablement (macOS only)
• Updated Display struct field usage in existing Linux code

Testing

The implementation has been thoroughly tested:

WindowManager wm;
auto windows = wm.GetAll();        // ✓ Works
auto current = wm.GetCurrent();    // ✓ Works  
auto window = wm.Get(12345);       // ✓ Works (returns nullptr for non-existent)

• ✅ Builds successfully on Linux
• ✅ Links correctly with all examples
• ✅ Handles headless environments safely
• ✅ All Window operations work without crashes

Compatibility

This implementation maintains full API compatibility with existing macOS/Windows versions while providing platform-appropriate behavior using GTK conventions.

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.