Persist advanced options expand/collapse state per section

Each ExpanderRow now remembers its expanded state between sessions
using a per-section key stored in SessionState. Replaces the global
detailed_mode toggle with granular per-section persistence.

pixstrip-gtk/src/steps/step_adjustments.rs

@@ -98,8 +98,16 @@ pub fn build_adjustments_page(state: &AppState) -> adw::NavigationPage {
         .title("Advanced Adjustments")
         .subtitle("Brightness, contrast, saturation, effects")
         .show_enable_switch(false)
+        .expanded(state.is_section_expanded("adjustments-advanced"))
         .build();
 
+    {
+        let st = state.clone();
+        adjust_expander.connect_expanded_notify(move |row| {
+            st.set_section_expanded("adjustments-advanced", row.is_expanded());
+        });
+    }
+
     // Brightness slider (-100 to +100)
     let brightness_row = adw::ActionRow::builder()
         .title("Brightness")