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_compress.rs

@@ -276,6 +276,53 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
     preview_group.add(&size_box);
     preview_group.add(&preview_frame);
 
+    // Thumbnail strip for selecting preview image
+    let thumb_scrolled = gtk::ScrolledWindow::builder()
+        .hscrollbar_policy(gtk::PolicyType::Automatic)
+        .vscrollbar_policy(gtk::PolicyType::Never)
+        .max_content_height(60)
+        .build();
+
+    let thumb_box = gtk::Box::builder()
+        .orientation(gtk::Orientation::Horizontal)
+        .spacing(4)
+        .margin_top(4)
+        .margin_bottom(4)
+        .halign(gtk::Align::Center)
+        .build();
+    thumb_scrolled.set_child(Some(&thumb_box));
+
+    let preview_index: Rc<RefCell<usize>> = Rc::new(RefCell::new(0));
+
+    // Populate thumbnails from loaded files
+    {
+        let files = state.loaded_files.borrow();
+        let max_thumbs = files.len().min(10); // Show at most 10 thumbnails
+        for i in 0..max_thumbs {
+            let pic = gtk::Picture::builder()
+                .content_fit(gtk::ContentFit::Cover)
+                .width_request(50)
+                .height_request(50)
+                .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();
+
+            thumb_box.append(&btn);
+        }
+        thumb_scrolled.set_visible(max_thumbs > 1);
+    }
+
     // "No image loaded" placeholder
     let no_image_label = gtk::Label::builder()
         .label("Add images first to see compression preview")
@@ -284,6 +331,7 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
         .margin_top(4)
         .build();
     preview_group.add(&no_image_label);
+    preview_group.add(&thumb_scrolled);
 
     content.append(&preview_group);
 
@@ -371,6 +419,7 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
         let comp_label = compressed_size_label.clone();
         let no_img_label = no_image_label.clone();
         let jc = state.job_config.clone();
+        let pidx = preview_index.clone();
 
         Rc::new(move || {
             let loaded = files.borrow();
@@ -380,8 +429,9 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
             }
             no_img_label.set_visible(false);
 
-            // Pick the first image as sample
+            // Pick the selected preview image
-            let sample_path = loaded[0].clone();
+            let idx = *pidx.borrow();
+            let sample_path = loaded.get(idx).cloned().unwrap_or_else(|| loaded[0].clone());
             let cfg = jc.borrow();
             let preset = cfg.quality_preset;
             drop(cfg);
@@ -449,6 +499,42 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
         })
     };
 
+    // Wire thumbnail buttons to switch preview image
+    {
+        let mut child = thumb_box.first_child();
+        let mut idx = 0usize;
+        while let Some(widget) = child {
+            if let Some(btn) = widget.downcast_ref::<gtk::Button>() {
+                let pidx = preview_index.clone();
+                let up = update_preview.clone();
+                let tb = thumb_box.clone();
+                let current_idx = idx;
+                btn.connect_clicked(move |_| {
+                    *pidx.borrow_mut() = current_idx;
+                    up();
+                    // Update highlight on thumbnails
+                    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;
+                    }
+                });
+                idx += 1;
+            }
+            child = widget.next_sibling();
+        }
+    }
+
     // Trigger initial preview load
     {
         let up = update_preview.clone();

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);