Add overwrite confirmation dialog and preset file drop import

- When overwrite behavior is "Ask", show dialog before processing
  listing conflicting files with Overwrite/Skip/Auto-rename options
- Workflow page accepts .pixstrip-preset file drag-and-drop for import
- Split run_processing into two phases to support async dialog

pixstrip-gtk/src/app.rs

@@ -1687,11 +1687,12 @@ fn run_processing(_window: &adw::ApplicationWindow, ui: &WizardUi) {
 
     job.preserve_directory_structure = cfg.preserve_dir_structure;
     job.output_dpi = cfg.output_dpi;
+    let ask_overwrite = cfg.overwrite_behavior == 0;
     job.overwrite_behavior = match cfg.overwrite_behavior {
         1 => pixstrip_core::operations::OverwriteBehavior::AutoRename,
         2 => pixstrip_core::operations::OverwriteBehavior::Overwrite,
         3 => pixstrip_core::operations::OverwriteBehavior::Skip,
-        _ => pixstrip_core::operations::OverwriteBehavior::AutoRename, // 0 "Ask" defaults to auto-rename in batch
+        _ => pixstrip_core::operations::OverwriteBehavior::AutoRename,
     };
 
     drop(cfg);
@@ -1700,6 +1701,66 @@ fn run_processing(_window: &adw::ApplicationWindow, ui: &WizardUi) {
         job.add_source(file);
     }
 
+    // Check for existing output files when "Ask" overwrite behavior is set
+    if ask_overwrite {
+        let output_dir = ui.state.output_dir.borrow().clone()
+            .unwrap_or_else(|| {
+                files[0].parent()
+                    .unwrap_or_else(|| std::path::Path::new("."))
+                    .join("processed")
+            });
+        if output_dir.exists() {
+            let conflicts: Vec<String> = files.iter()
+                .filter_map(|f| {
+                    let name = f.file_name()?;
+                    let out_path = output_dir.join(name);
+                    if out_path.exists() { Some(name.to_string_lossy().into()) } else { None }
+                })
+                .take(10)
+                .collect();
+
+            if !conflicts.is_empty() {
+                let msg = if conflicts.len() == 10 {
+                    format!("{} files already exist in the output folder (showing first 10):\n\n{}", conflicts.len(), conflicts.join("\n"))
+                } else {
+                    format!("{} files already exist in the output folder:\n\n{}", conflicts.len(), conflicts.join("\n"))
+                };
+
+                let confirm = adw::AlertDialog::builder()
+                    .heading("Files Already Exist")
+                    .body(&msg)
+                    .build();
+                confirm.add_response("rename", "Auto-rename");
+                confirm.add_response("skip", "Skip Existing");
+                confirm.add_response("overwrite", "Overwrite All");
+                confirm.set_response_appearance("overwrite", adw::ResponseAppearance::Destructive);
+                confirm.set_default_response(Some("rename"));
+                confirm.set_close_response("rename");
+
+                let ui_c = ui.clone();
+                let window_c = _window.clone();
+                let mut job_c = job;
+                confirm.choose(Some(_window), gtk::gio::Cancellable::NONE, move |response| {
+                    job_c.overwrite_behavior = match response.as_str() {
+                        "overwrite" => pixstrip_core::operations::OverwriteBehavior::Overwrite,
+                        "skip" => pixstrip_core::operations::OverwriteBehavior::Skip,
+                        _ => pixstrip_core::operations::OverwriteBehavior::AutoRename,
+                    };
+                    continue_processing(&window_c, &ui_c, job_c);
+                });
+                return;
+            }
+        }
+    }
+
+    continue_processing(_window, ui, job);
+}
+
+fn continue_processing(
+    _window: &adw::ApplicationWindow,
+    ui: &WizardUi,
+    job: pixstrip_core::pipeline::ProcessingJob,
+) {
     // Build processing UI inline in the nav_view
     let processing_page = crate::processing::build_processing_page();
     ui.nav_view.push(&processing_page);