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/core/analysis.rs

@@ -117,6 +117,11 @@ pub fn run_background_analysis(id: i64, path: PathBuf, appimage_type: AppImageTy
         } else {
             Some(serde_json::to_string(&meta.desktop_actions).unwrap_or_default())
         };
+        let screenshot_urls_str = if meta.screenshot_urls.is_empty() {
+            None
+        } else {
+            Some(meta.screenshot_urls.join("\n"))
+        };
 
         if let Err(e) = db.update_appstream_metadata(
             id,
@@ -136,6 +141,7 @@ pub fn run_background_analysis(id: i64, path: PathBuf, appimage_type: AppImageTy
             release_json.as_deref(),
             actions_json.as_deref(),
             meta.has_signature,
+            screenshot_urls_str.as_deref(),
         ) {
             log::warn!("Failed to update appstream metadata for id {}: {}", id, e);
         }