Fix performance, add screenshots, make banner scrollable

- Make detail view banner scroll with content instead of staying fixed,
  preventing tall banners from eating screen space
- Optimize squashfs offset scanning with buffered 256KB chunk reading
  instead of loading entire file into memory (critical for 1.5GB+ files)
- Add screenshot URL parsing from AppStream XML and async image display
  with carousel in the overview tab
- Fix infinite re-analysis bug: has_appstream check caused every app
  without AppStream data to be re-analyzed on every startup. Now handled
  via one-time migration reset in v10
- Database migration v10: add screenshot_urls column, reset analysis
  status for one-time re-scan with new parser

src/ui/detail_view.rs

@@ -1,5 +1,6 @@
 use adw::prelude::*;
 use std::cell::Cell;
+use std::io::Read as _;
 use std::rc::Rc;
 
 use gtk::gio;
@@ -42,20 +43,19 @@ pub fn build_detail_page(record: &AppImageRecord, db: &Rc<Database>) -> adw::Nav
     view_stack.add_titled(&storage_page, Some("storage"), "Storage");
     view_stack.page(&storage_page).set_icon_name(Some("drive-harddisk-symbolic"));
 
-    // Scrollable view stack
+    // Banner scrolls with content (not sticky) so tall banners don't eat space
+    let scroll_content = gtk::Box::builder()
+        .orientation(gtk::Orientation::Vertical)
+        .build();
+    scroll_content.append(&build_banner(record));
+    scroll_content.append(&view_stack);
+
     let scrolled = gtk::ScrolledWindow::builder()
-        .child(&view_stack)
+        .child(&scroll_content)
         .vexpand(true)
         .build();
 
-    // Main vertical layout: banner + scrolled tabs
-    let content = gtk::Box::builder()
-        .orientation(gtk::Orientation::Vertical)
-        .build();
-    content.append(&build_banner(record));
-    content.append(&scrolled);
-
-    toast_overlay.set_child(Some(&content));
+    toast_overlay.set_child(Some(&scrolled));
 
     // Header bar with ViewSwitcher as title widget (standard GNOME pattern)
     let header = adw::HeaderBar::new();
@@ -366,6 +366,93 @@ fn build_overview_tab(record: &AppImageRecord, db: &Rc<Database>) -> gtk::Box {
         }
     }
 
+    // -----------------------------------------------------------------------
+    // Screenshots section - async image loading from URLs
+    // -----------------------------------------------------------------------
+    if let Some(ref urls_str) = record.screenshot_urls {
+        let urls: Vec<&str> = urls_str.lines().filter(|u| !u.is_empty()).collect();
+        if !urls.is_empty() {
+            let screenshots_group = adw::PreferencesGroup::builder()
+                .title("Screenshots")
+                .build();
+
+            let carousel = adw::Carousel::builder()
+                .hexpand(true)
+                .allow_scroll_wheel(true)
+                .allow_mouse_drag(true)
+                .build();
+            carousel.set_height_request(300);
+
+            let dots = adw::CarouselIndicatorDots::builder()
+                .carousel(&carousel)
+                .build();
+
+            for url in &urls {
+                let picture = gtk::Picture::builder()
+                    .content_fit(gtk::ContentFit::Contain)
+                    .height_request(300)
+                    .build();
+                picture.set_can_shrink(true);
+
+                // Placeholder spinner while loading
+                let overlay = gtk::Overlay::builder().child(&picture).build();
+                let spinner = adw::Spinner::builder()
+                    .width_request(32)
+                    .height_request(32)
+                    .halign(gtk::Align::Center)
+                    .valign(gtk::Align::Center)
+                    .build();
+                overlay.add_overlay(&spinner);
+
+                carousel.append(&overlay);
+
+                // Load image asynchronously
+                let url_owned = url.to_string();
+                let picture_ref = picture.clone();
+                let spinner_ref = spinner.clone();
+                glib::spawn_future_local(async move {
+                    let result = gio::spawn_blocking(move || {
+                        let mut response = ureq::get(&url_owned)
+                            .header("User-Agent", "Driftwood-AppImage-Manager/0.1")
+                            .call()
+                            .ok()?;
+                        let mut buf = Vec::new();
+                        response.body_mut().as_reader().read_to_end(&mut buf).ok()?;
+                        Some(buf)
+                    })
+                    .await;
+
+                    spinner_ref.set_visible(false);
+                    if let Ok(Some(data)) = result {
+                        let gbytes = glib::Bytes::from(&data);
+                        let stream = gio::MemoryInputStream::from_bytes(&gbytes);
+                        if let Ok(pixbuf) = gtk::gdk_pixbuf::Pixbuf::from_stream(
+                            &stream,
+                            None::<&gio::Cancellable>,
+                        ) {
+                            let texture = gtk::gdk::Texture::for_pixbuf(&pixbuf);
+                            picture_ref.set_paintable(Some(&texture));
+                        }
+                    }
+                });
+            }
+
+            let carousel_box = gtk::Box::builder()
+                .orientation(gtk::Orientation::Vertical)
+                .spacing(8)
+                .margin_top(8)
+                .margin_bottom(8)
+                .build();
+            carousel_box.append(&carousel);
+            if urls.len() > 1 {
+                carousel_box.append(&dots);
+            }
+            screenshots_group.add(&carousel_box);
+
+            inner.append(&screenshots_group);
+        }
+    }
+
     // -----------------------------------------------------------------------
     // Links section
     // -----------------------------------------------------------------------