Wire skill level and accessibility settings to UI

- Add detailed_mode to AppState, derived from skill_level setting
- Expand advanced option sections by default in Detailed mode
  (resize, convert, compress, watermark steps)
- Fix high contrast to use HighContrast GTK theme
- Add completion sound via canberra-gtk-play

pixstrip-gtk/src/steps/step_watermark.rs

@@ -176,6 +176,7 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         .title("Advanced Options")
         .subtitle("Opacity, rotation, tiling, margin")
         .show_enable_switch(false)
+        .expanded(state.detailed_mode)
         .build();
 
     let opacity_row = adw::SpinRow::builder()