[WIP] [Linux] Implement Window class

[Copilot]

Plan to implement Linux Window class using GTK+3:

• Analyze repository structure and build system
• Understand macOS Window implementation pattern (uses pimpl idiom)
• Configure Linux build environment (GTK+3)
• Create src/platform/linux/window_linux.cpp implementation file
• Implement Window::Impl class using GtkWindow
• Implement constructors and destructor
• Implement focus and visibility methods
• Implement window state methods (maximized, minimized, fullscreen)
• Implement window properties (bounds, size, position, title)
• Implement window behavior methods (resizable, movable, etc.)
• Create src/platform/linux/window_manager_linux.cpp if needed
• Test the implementation
• Build and validate changes

Note: There are existing build errors in display_manager_linux.cpp that are unrelated to this Window implementation task.

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