Add single-instance file handling via GIO HANDLES_OPEN

- App flag HANDLES_OPEN enables receiving files from file manager
- connect_open handler filters for image files and activates window
- load-external-files action receives paths and adds to loaded files
- Auto-navigates to step 2 (images) when files arrive externally
- Second instance sends files to existing window instead of launching

pixstrip-gtk/src/app.rs

@@ -141,9 +141,55 @@ struct WizardUi {
 pub fn build_app() -> adw::Application {
     let app = adw::Application::builder()
         .application_id(APP_ID)
+        .flags(gtk::gio::ApplicationFlags::HANDLES_OPEN)
         .build();
 
     app.connect_activate(build_ui);
+
+    // Handle files opened via file manager integration or command line
+    app.connect_open(|app, files, _hint| {
+        // Ensure the window exists (activate first if needed)
+        if app.active_window().is_none() {
+            app.activate();
+        }
+
+        // Collect valid image file paths
+        let image_paths: Vec<std::path::PathBuf> = files.iter()
+            .filter_map(|f| f.path())
+            .filter(|p| {
+                p.extension()
+                    .and_then(|e| e.to_str())
+                    .is_some_and(|ext| {
+                        matches!(
+                            ext.to_lowercase().as_str(),
+                            "jpg" | "jpeg" | "png" | "webp" | "avif" | "gif" | "tiff" | "tif" | "bmp"
+                        )
+                    })
+            })
+            .collect();
+
+        if image_paths.is_empty() {
+            return;
+        }
+
+        // Find the WizardUi through the window's action group
+        if let Some(window) = app.active_window() {
+            // Store files in a global channel for the UI to pick up
+            if let Some(action) = window.downcast_ref::<adw::ApplicationWindow>()
+                .and_then(|w| w.lookup_action("load-external-files"))
+            {
+                // Serialize paths as string variant
+                let paths_str: String = image_paths.iter()
+                    .map(|p| p.display().to_string())
+                    .collect::<Vec<_>>()
+                    .join("\n");
+                action.downcast_ref::<gtk::gio::SimpleAction>()
+                    .unwrap()
+                    .activate(Some(&paths_str.to_variant()));
+            }
+        }
+    });
+
     setup_shortcuts(&app);
 
     // App-level quit action
@@ -846,6 +892,39 @@ fn setup_window_actions(window: &adw::ApplicationWindow, ui: &WizardUi) {
         }
     });
 
+    // Load external files action - used by single-instance file open
+    {
+        let ui = ui.clone();
+        let action = gtk::gio::SimpleAction::new("load-external-files", Some(&String::static_variant_type()));
+        action.connect_activate(move |_, param| {
+            if let Some(paths_str) = param.and_then(|v| v.get::<String>()) {
+                let new_files: Vec<std::path::PathBuf> = paths_str
+                    .lines()
+                    .map(std::path::PathBuf::from)
+                    .filter(|p| p.exists())
+                    .collect();
+
+                if !new_files.is_empty() {
+                    let count = new_files.len();
+                    ui.state.loaded_files.borrow_mut().extend(new_files);
+                    ui.toast_overlay.add_toast(adw::Toast::new(
+                        &format!("{} images added from file manager", count)
+                    ));
+
+                    // Navigate to step 2 (images) if we're on step 1 or earlier
+                    let current = ui.state.wizard.borrow().current_step;
+                    if current <= 1 {
+                        ui.state.wizard.borrow_mut().current_step = 1;
+                        if let Some(page) = ui.pages.get(1) {
+                            ui.nav_view.push(page);
+                        }
+                    }
+                }
+            }
+        });
+        action_group.add_action(&action);
+    }
+
     window.insert_action_group("win", Some(&action_group));
 }