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/integrator.rs

@@ -285,6 +285,7 @@ mod tests {
             release_history: None,
             desktop_actions: None,
             has_signature: false,
+            screenshot_urls: None,
         };
 
         // We can't easily test the full integrate() without mocking dirs,