Add visual size preview to resize step

Show proportional rectangles comparing original image dimensions
(gray) vs target output dimensions (blue). Preview updates live
as the user changes width/height values. Uses first loaded image
for actual dimensions when available.

pixstrip-gtk/src/steps/step_resize.rs

@@ -224,6 +224,110 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
 
     content.append(&mode_stack);
 
+    // Size preview visualization
+    let preview_group = adw::PreferencesGroup::builder()
+        .title("Size Preview")
+        .description("Visual comparison of original and output dimensions")
+        .build();
+
+    // Use shared state for the preview drawing
+    let preview_width = std::rc::Rc::new(std::cell::RefCell::new(cfg.resize_width));
+    let preview_height = std::rc::Rc::new(std::cell::RefCell::new(cfg.resize_height));
+    let loaded_files = state.loaded_files.clone();
+
+    let drawing = gtk::DrawingArea::builder()
+        .content_width(300)
+        .content_height(150)
+        .halign(gtk::Align::Center)
+        .margin_top(8)
+        .margin_bottom(8)
+        .build();
+
+    let pw = preview_width.clone();
+    let ph = preview_height.clone();
+    let lf = loaded_files.clone();
+    drawing.set_draw_func(move |_area, cr, width, height| {
+        let target_w = *pw.borrow() as f64;
+        let target_h = *ph.borrow() as f64;
+
+        // Try to get actual first image dimensions
+        let (orig_w, orig_h) = {
+            let files = lf.borrow();
+            if let Some(first) = files.first() {
+                image_dimensions(first).unwrap_or((4000.0, 3000.0))
+            } else {
+                (4000.0, 3000.0)
+            }
+        };
+
+        let actual_target_w = if target_w > 0.0 { target_w } else { orig_w };
+        let actual_target_h = if target_h > 0.0 {
+            target_h
+        } else if target_w > 0.0 {
+            orig_h * target_w / orig_w
+        } else {
+            orig_h
+        };
+
+        let max_dim = orig_w.max(orig_h).max(actual_target_w).max(actual_target_h);
+        if max_dim == 0.0 {
+            return;
+        }
+
+        let pad = 20.0;
+        let avail_w = width as f64 - pad * 2.0;
+        let avail_h = height as f64 - pad * 2.0;
+        let scale = (avail_w / max_dim).min(avail_h / max_dim);
+
+        let ow = orig_w * scale;
+        let oh = orig_h * scale;
+        let tw = actual_target_w * scale;
+        let th = actual_target_h * scale;
+
+        // Draw original rectangle (semi-transparent)
+        let _ = cr.set_source_rgba(0.5, 0.5, 0.5, 0.3);
+        let _ = cr.rectangle(pad, pad, ow, oh);
+        let _ = cr.fill();
+        let _ = cr.set_source_rgba(0.5, 0.5, 0.5, 0.7);
+        let _ = cr.rectangle(pad, pad, ow, oh);
+        let _ = cr.stroke();
+
+        // Draw target rectangle (accent color)
+        let _ = cr.set_source_rgba(0.2, 0.5, 0.9, 0.3);
+        let _ = cr.rectangle(pad, pad, tw, th);
+        let _ = cr.fill();
+        let _ = cr.set_source_rgba(0.2, 0.5, 0.9, 0.9);
+        let _ = cr.rectangle(pad, pad, tw, th);
+        let _ = cr.stroke();
+
+        // Labels
+        let _ = cr.set_source_rgba(0.6, 0.6, 0.6, 1.0);
+        let _ = cr.set_font_size(10.0);
+        let _ = cr.move_to(pad + ow + 4.0, pad + oh / 2.0);
+        let _ = cr.show_text(&format!("{}x{}", orig_w as u32, orig_h as u32));
+
+        let _ = cr.set_source_rgba(0.2, 0.5, 0.9, 1.0);
+        let _ = cr.move_to(pad + tw + 4.0, pad + th / 2.0 + 12.0);
+        let _ = cr.show_text(&format!("{}x{}", actual_target_w as u32, actual_target_h as u32));
+    });
+
+    let preview_label = gtk::Label::builder()
+        .label("Gray = original, Blue = target size")
+        .css_classes(["dim-label", "caption"])
+        .halign(gtk::Align::Center)
+        .build();
+
+    let preview_box = gtk::Box::builder()
+        .orientation(gtk::Orientation::Vertical)
+        .spacing(4)
+        .margin_top(4)
+        .margin_bottom(4)
+        .build();
+    preview_box.append(&drawing);
+    preview_box.append(&preview_label);
+    preview_group.add(&preview_box);
+    content.append(&preview_group);
+
     // Basic orientation adjustments (folded into resize step per design doc)
     let orientation_group = adw::PreferencesGroup::builder()
         .title("Orientation")
@@ -308,14 +412,24 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
     }
     {
         let jc = state.job_config.clone();
+        let pw = preview_width.clone();
+        let draw = drawing.clone();
         width_row.connect_value_notify(move |row| {
-            jc.borrow_mut().resize_width = row.value() as u32;
+            let val = row.value() as u32;
+            jc.borrow_mut().resize_width = val;
+            *pw.borrow_mut() = val;
+            draw.queue_draw();
         });
     }
     {
         let jc = state.job_config.clone();
+        let ph = preview_height.clone();
+        let draw = drawing.clone();
         height_row.connect_value_notify(move |row| {
-            jc.borrow_mut().resize_height = row.value() as u32;
+            let val = row.value() as u32;
+            jc.borrow_mut().resize_height = val;
+            *ph.borrow_mut() = val;
+            draw.queue_draw();
         });
     }
     {
@@ -350,3 +464,11 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
         .child(&clamp)
         .build()
 }
+
+/// Get dimensions of an image file without loading the full image
+fn image_dimensions(path: &std::path::Path) -> Option<(f64, f64)> {
+    let reader = image::ImageReader::open(path).ok()?;
+    let reader = reader.with_guessed_format().ok()?;
+    let (w, h) = reader.into_dimensions().ok()?;
+    Some((w as f64, h as f64))
+}