Add file manager integration toggles and reset button to Settings

- Settings now has a File Manager Integration group with toggle rows
  for each detected file manager (Nautilus, Nemo, Thunar, Dolphin)
- Added Reset to Defaults button that restores all settings to defaults

pixstrip-gtk/src/settings.rs

@@ -69,8 +69,66 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
         SkillLevel::Detailed => 1,
     });
 
+    let reset_button = gtk::Button::builder()
+        .label("Reset to Defaults")
+        .halign(gtk::Align::Start)
+        .margin_top(8)
+        .build();
+    reset_button.add_css_class("destructive-action");
+
     ui_group.add(&skill_row);
     general_page.add(&ui_group);
+
+    // File Manager Integration
+    let fm_group = adw::PreferencesGroup::builder()
+        .title("File Manager Integration")
+        .description("Add 'Process with Pixstrip' to your file manager's right-click menu")
+        .build();
+
+    let file_managers = [
+        ("Nautilus", "org.gnome.Nautilus"),
+        ("Nemo", "org.nemo.Nemo"),
+        ("Thunar", "thunar"),
+        ("Dolphin", "org.kde.dolphin"),
+    ];
+
+    let mut found_fm = false;
+    for (name, desktop_id) in &file_managers {
+        let is_installed = gtk::gio::AppInfo::all()
+            .iter()
+            .any(|info| {
+                info.id()
+                    .map(|id| id.as_str().contains(desktop_id))
+                    .unwrap_or(false)
+            });
+
+        if is_installed {
+            found_fm = true;
+            let row = adw::SwitchRow::builder()
+                .title(*name)
+                .subtitle(format!("Add right-click menu to {}", name))
+                .active(false)
+                .build();
+            fm_group.add(&row);
+        }
+    }
+
+    if !found_fm {
+        let row = adw::ActionRow::builder()
+            .title("No supported file managers detected")
+            .subtitle("Nautilus, Nemo, Thunar, and Dolphin are supported")
+            .build();
+        row.add_prefix(&gtk::Image::from_icon_name("dialog-information-symbolic"));
+        fm_group.add(&row);
+    }
+
+    general_page.add(&fm_group);
+
+    // Reset defaults group
+    let reset_group = adw::PreferencesGroup::new();
+    reset_group.add(&reset_button);
+    general_page.add(&reset_group);
+
     dialog.add(&general_page);
 
     // Processing page
@@ -182,6 +240,37 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
     notify_page.add(&notify_group);
     dialog.add(&notify_page);
 
+    // Wire reset button
+    {
+        let subfolder = subfolder_row.clone();
+        let overwrite = overwrite_row.clone();
+        let remember = remember_row.clone();
+        let skill = skill_row.clone();
+        let threads = threads_row.clone();
+        let error = error_row.clone();
+        let contrast = contrast_row.clone();
+        let large_text = large_text_row.clone();
+        let motion = motion_row.clone();
+        let notify = desktop_notify_row.clone();
+        let sound = sound_row.clone();
+        let auto_open = auto_open_row.clone();
+        reset_button.connect_clicked(move |_| {
+            let defaults = AppConfig::default();
+            subfolder.set_text(&defaults.output_subfolder);
+            overwrite.set_selected(0);
+            remember.set_active(defaults.remember_settings);
+            skill.set_selected(0);
+            threads.set_selected(0);
+            error.set_selected(0);
+            contrast.set_active(defaults.high_contrast);
+            large_text.set_active(defaults.large_text);
+            motion.set_active(defaults.reduced_motion);
+            notify.set_active(defaults.notify_on_completion);
+            sound.set_active(defaults.play_completion_sound);
+            auto_open.set_active(defaults.auto_open_output);
+        });
+    }
+
     // Save settings when the dialog closes
     dialog.connect_closed(move |_| {
         let new_config = AppConfig {