feat: add real-time PPT page turn button preview in settings

- Add 16:9 preview area showing actual button layout
- Preview updates in real-time based on settings:
  - Toggle visibility per position (Left/Right/LeftBottom/RightBottom)
  - Opacity sliders affect preview transparency
  - Offset sliders move preview buttons accordingly
  - Page number display toggles
- Replace CheckBox with ToggleSwitch for position toggles
- Add reset buttons for offset and opacity sliders
- Match preview visual style with actual PptNavBar control

Author: PANDAJSR