Add thumbnail selection for compression and watermark previews

Users can now click different batch images in a thumbnail strip to
switch which image is used for the quality comparison preview and
watermark position preview. Shows up to 10 thumbnails with accent
highlight on the selected one.

pixstrip-gtk/src/steps/step_watermark.rs

@@ -237,6 +237,59 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         preview_picture.set_visible(has_files);
     }
 
+    // Thumbnail strip for selecting preview image
+    let wm_thumb_box = gtk::Box::builder()
+        .orientation(gtk::Orientation::Horizontal)
+        .spacing(4)
+        .halign(gtk::Align::Center)
+        .margin_top(4)
+        .build();
+    {
+        let files = state.loaded_files.borrow();
+        let max_thumbs = files.len().min(10);
+        for i in 0..max_thumbs {
+            let pic = gtk::Picture::builder()
+                .content_fit(gtk::ContentFit::Cover)
+                .width_request(40)
+                .height_request(40)
+                .build();
+            pic.set_filename(Some(&files[i]));
+            let frame = gtk::Frame::builder()
+                .child(&pic)
+                .build();
+            if i == 0 { frame.add_css_class("accent"); }
+
+            let btn = gtk::Button::builder()
+                .child(&frame)
+                .has_frame(false)
+                .tooltip_text(files[i].file_name().and_then(|n| n.to_str()).unwrap_or("image"))
+                .build();
+
+            let pp = preview_picture.clone();
+            let path = files[i].clone();
+            let tb = wm_thumb_box.clone();
+            let current_idx = i;
+            btn.connect_clicked(move |_| {
+                pp.set_filename(Some(&path));
+                let mut c = tb.first_child();
+                let mut j = 0usize;
+                while let Some(w) = c {
+                    if let Some(b) = w.downcast_ref::<gtk::Button>() {
+                        if let Some(f) = b.child().and_then(|c| c.downcast::<gtk::Frame>().ok()) {
+                            if j == current_idx { f.add_css_class("accent"); }
+                            else { f.remove_css_class("accent"); }
+                        }
+                    }
+                    c = w.next_sibling();
+                    j += 1;
+                }
+            });
+
+            wm_thumb_box.append(&btn);
+        }
+        wm_thumb_box.set_visible(max_thumbs > 1);
+    }
+
     let preview_stack = gtk::Box::builder()
         .orientation(gtk::Orientation::Vertical)
         .spacing(4)
@@ -244,6 +297,7 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         .margin_bottom(8)
         .build();
     preview_stack.append(&preview_overlay);
+    preview_stack.append(&wm_thumb_box);
     preview_stack.append(&no_preview_label);
 
     preview_group.add(&preview_stack);