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
         });

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")

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")

pixstrip-gtk/src/steps/step_convert.rs

@@ -145,9 +145,16 @@ 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)
+        .expanded(state.is_section_expanded("convert-advanced"))
         .build();
 
+    {
+        let st = state.clone();
+        advanced_expander.connect_expanded_notify(move |row| {
+            st.set_section_expanded("convert-advanced", row.is_expanded());
+        });
+    }
+
     let progressive_row = adw::SwitchRow::builder()
         .title("Progressive JPEG")
         .subtitle("Loads gradually in browsers, slightly larger")

pixstrip-gtk/src/steps/step_resize.rs

@@ -372,9 +372,16 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
         .title("Advanced Options")
         .subtitle("Resize algorithm, DPI, upscale behavior")
         .show_enable_switch(false)
-        .expanded(state.detailed_mode)
+        .expanded(state.is_section_expanded("resize-advanced"))
         .build();
 
+    {
+        let st = state.clone();
+        advanced_expander.connect_expanded_notify(move |row| {
+            st.set_section_expanded("resize-advanced", row.is_expanded());
+        });
+    }
+
     let upscale_row = adw::SwitchRow::builder()
         .title("Allow Upscaling")
         .subtitle("Enlarge images smaller than target size")

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")