Add fixed output folder option in settings

Users can now choose between subfolder-next-to-originals or a fixed
output folder in Settings > General. The fixed path is selectable via
a folder picker dialog and persisted across sessions.

pixstrip-gtk/src/settings.rs

@@ -20,11 +20,84 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
         .title("Output")
         .build();
 
+    // Output mode: subfolder or fixed path
+    let output_mode_row = adw::ComboRow::builder()
+        .title("Default output location")
+        .subtitle("Where processed images are saved by default")
+        .build();
+    let output_mode_model = gtk::StringList::new(&[
+        "Subfolder next to originals",
+        "Fixed output folder",
+    ]);
+    output_mode_row.set_model(Some(&output_mode_model));
+    output_mode_row.set_selected(if config.output_fixed_path.is_some() { 1 } else { 0 });
+
     let subfolder_row = adw::EntryRow::builder()
         .title("Default output subfolder")
         .text(&config.output_subfolder)
+        .visible(config.output_fixed_path.is_none())
         .build();
 
+    let fixed_path_row = adw::ActionRow::builder()
+        .title("Fixed output folder")
+        .subtitle(
+            config.output_fixed_path
+                .as_deref()
+                .unwrap_or("No folder selected"),
+        )
+        .activatable(true)
+        .visible(config.output_fixed_path.is_some())
+        .build();
+    fixed_path_row.add_prefix(&gtk::Image::from_icon_name("folder-open-symbolic"));
+
+    let choose_fixed_btn = gtk::Button::builder()
+        .icon_name("document-open-symbolic")
+        .tooltip_text("Choose output folder")
+        .valign(gtk::Align::Center)
+        .build();
+    choose_fixed_btn.add_css_class("flat");
+    fixed_path_row.add_suffix(&choose_fixed_btn);
+
+    // Shared state for fixed path
+    let fixed_path_state: std::rc::Rc<std::cell::RefCell<Option<String>>> =
+        std::rc::Rc::new(std::cell::RefCell::new(config.output_fixed_path.clone()));
+
+    // Wire output mode toggle
+    {
+        let sf = subfolder_row.clone();
+        let fp = fixed_path_row.clone();
+        output_mode_row.connect_selected_notify(move |row| {
+            let is_fixed = row.selected() == 1;
+            sf.set_visible(!is_fixed);
+            fp.set_visible(is_fixed);
+        });
+    }
+
+    // Wire fixed path chooser
+    {
+        let fps = fixed_path_state.clone();
+        let fpr = fixed_path_row.clone();
+        choose_fixed_btn.connect_clicked(move |btn| {
+            let fps = fps.clone();
+            let fpr = fpr.clone();
+            let dialog = gtk::FileDialog::builder()
+                .title("Choose Output Folder")
+                .modal(true)
+                .build();
+            if let Some(window) = btn.root().and_then(|r| r.downcast::<gtk::Window>().ok()) {
+                dialog.select_folder(Some(&window), gtk::gio::Cancellable::NONE, move |result| {
+                    if let Ok(file) = result
+                        && let Some(path) = file.path()
+                    {
+                        let path_str = path.display().to_string();
+                        fpr.set_subtitle(&path_str);
+                        *fps.borrow_mut() = Some(path_str);
+                    }
+                });
+            }
+        });
+    }
+
     let overwrite_row = adw::ComboRow::builder()
         .title("Default overwrite behavior")
         .subtitle("What to do when output files already exist")
@@ -49,7 +122,9 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
         .active(config.remember_settings)
         .build();
 
+    output_group.add(&output_mode_row);
     output_group.add(&subfolder_row);
+    output_group.add(&fixed_path_row);
     output_group.add(&overwrite_row);
     output_group.add(&remember_row);
     general_page.add(&output_group);
@@ -366,6 +441,8 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
         let notify = desktop_notify_row.clone();
         let sound = sound_row.clone();
         let auto_open = auto_open_row.clone();
+        let output_mode = output_mode_row.clone();
+        let fps_reset = fixed_path_state.clone();
         reset_button.connect_clicked(move |_| {
             let defaults = AppConfig::default();
             subfolder.set_text(&defaults.output_subfolder);
@@ -380,6 +457,8 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
             notify.set_active(defaults.notify_on_completion);
             sound.set_active(defaults.play_completion_sound);
             auto_open.set_active(defaults.auto_open_output);
+            output_mode.set_selected(0);
+            *fps_reset.borrow_mut() = None;
         });
     }
 
@@ -393,7 +472,11 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
             first_run_complete: true,
             tutorial_complete: true, // preserve if settings are being saved
             output_subfolder: subfolder_row.text().to_string(),
-            output_fixed_path: None,
+            output_fixed_path: if output_mode_row.selected() == 1 {
+                fixed_path_state.borrow().clone()
+            } else {
+                None
+            },
             overwrite_behavior: match overwrite_row.selected() {
                 1 => OverwriteBehavior::AutoRename,
                 2 => OverwriteBehavior::Overwrite,