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/app.rs

@@ -102,6 +102,19 @@ pub struct AppState {
     pub job_config: Rc<RefCell<JobConfig>>,
     pub detailed_mode: bool,
     pub batch_queue: Rc<RefCell<BatchQueue>>,
+    pub expanded_sections: Rc<RefCell<std::collections::HashMap<String, bool>>>,
+}
+
+impl AppState {
+    /// Get the expanded state for a named section, falling back to the skill level default
+    pub fn is_section_expanded(&self, section: &str) -> bool {
+        self.expanded_sections.borrow().get(section).copied().unwrap_or(self.detailed_mode)
+    }
+
+    /// Set and persist an expander's state
+    pub fn set_section_expanded(&self, section: &str, expanded: bool) {
+        self.expanded_sections.borrow_mut().insert(section.to_string(), expanded);
+    }
 }
 
 /// A queued batch of images with their processing settings
@@ -300,6 +313,7 @@ fn build_ui(app: &adw::Application) {
         output_dir: Rc::new(RefCell::new(None)),
         detailed_mode: app_cfg.skill_level.is_advanced(),
         batch_queue: Rc::new(RefCell::new(BatchQueue::default())),
+        expanded_sections: Rc::new(RefCell::new(sess_state.expanded_sections.clone())),
         job_config: Rc::new(RefCell::new(JobConfig {
             resize_enabled: if remember { sess_state.resize_enabled.unwrap_or(true) } else { true },
             resize_width: if remember { sess_state.resize_width.unwrap_or(1200) } else { 1200 },
@@ -574,6 +588,9 @@ fn build_ui(app: &adw::Application) {
                 state.rename_enabled = Some(cfg.rename_enabled);
             }
 
+            // Always save expanded section states
+            state.expanded_sections = app_state_for_close.expanded_sections.borrow().clone();
+
             let _ = session.save(&state);
             glib::Propagation::Proceed
         });