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_compress.rs

@@ -343,9 +343,16 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
     let advanced_expander = adw::ExpanderRow::builder()
         .title("Per-Format Quality")
         .subtitle("Fine-tune quality for each format individually")
-        .expanded(state.detailed_mode)
+        .expanded(state.is_section_expanded("compress-advanced"))
         .build();
 
+    {
+        let st = state.clone();
+        advanced_expander.connect_expanded_notify(move |row| {
+            st.set_section_expanded("compress-advanced", row.is_expanded());
+        });
+    }
+
     let jpeg_row = adw::SpinRow::builder()
         .title("JPEG Quality")
         .subtitle("1-100, higher is better quality")