Fix critical bugs and improve stability across multiple components
- Fix circular property binding in Main.qml
- Correct SDDM function calls in PowerMenu.qml (reboot/powerOff)
- Add array bounds checking for keyboard layouts in MenuArea.qml
- Add division by zero protection in UserSelector.qml
- Add division by zero protection in SessionSelector.qml
- Add Canvas zero dimension protection in Avatar.qml
- Fix height calculation edge cases in LockScreen.qml
- Add comprehensive keyboard null checks in LayoutSelector.qml
- Fix scope violation issues in VirtualKeyboard.qml
- Add parameter null safety in Languages.qml
- Improve string safety in LoginScreen.qml
These fixes prevent crashes and improve stability in edge cases.