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_convert.rs

@@ -145,6 +145,7 @@ pub fn build_convert_page(state: &AppState) -> adw::NavigationPage {
         .title("Format Mapping")
         .subtitle("Different input formats can convert to different outputs")
         .show_enable_switch(false)
+        .expanded(state.detailed_mode)
         .build();
 
     let progressive_row = adw::SwitchRow::builder()