Fix native titlebar dragging on macOS

[KCKT0112]

Summary

• add a macOS window attribute, native-titlebar-draggable, to restore dragging from the native titlebar area
• use performWindowDragWithEvent: for left mouse down events in the native titlebar while preserving traffic-light button clicks
• reapply the setting when the NSWindow becomes available

Notes

• Default behavior is unchanged; the new handling is only enabled when the attribute is set to true.
• Handles both flipped and non-flipped NSView coordinate systems.

Testing

• Built QWKCore/QWKQuick as part of the downstream Qt 6.9.3 app build on macOS
• Verified the downstream app can drag from the native traffic-light titlebar area without making the custom header draggable