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_watermark.rs

@@ -336,9 +336,16 @@ 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)
+        .expanded(state.is_section_expanded("watermark-advanced"))
         .build();
 
+    {
+        let st = state.clone();
+        advanced_expander.connect_expanded_notify(move |row| {
+            st.set_section_expanded("watermark-advanced", row.is_expanded());
+        });
+    }
+
     // Text color picker
     let color_row = adw::ActionRow::builder()
         .title("Text Color")