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/database.rs

@@ -67,6 +67,7 @@ pub struct AppImageRecord {
     pub release_history: Option<String>,
     pub desktop_actions: Option<String>,
     pub has_signature: bool,
+    pub screenshot_urls: Option<String>,
 }
 
 #[derive(Debug, Clone)]
@@ -364,6 +365,10 @@ impl Database {
             self.migrate_to_v9()?;
         }
 
+        if current_version < 10 {
+            self.migrate_to_v10()?;
+        }
+
         // Ensure all expected columns exist (repairs DBs where a migration
         // was updated after it had already run on this database)
         self.ensure_columns()?;
@@ -741,6 +746,30 @@ impl Database {
         Ok(())
     }
 
+    fn migrate_to_v10(&self) -> SqlResult<()> {
+        let sql = "ALTER TABLE appimages ADD COLUMN screenshot_urls TEXT";
+        match self.conn.execute(sql, []) {
+            Ok(_) => {}
+            Err(e) => {
+                let msg = e.to_string();
+                if !msg.contains("duplicate column") {
+                    return Err(e);
+                }
+            }
+        }
+        // Force one-time re-analysis so the new AppStream parser (screenshots,
+        // extended metadata) runs on existing apps
+        self.conn.execute(
+            "UPDATE appimages SET analysis_status = NULL WHERE analysis_status = 'complete'",
+            [],
+        )?;
+        self.conn.execute(
+            "UPDATE schema_version SET version = ?1",
+            params![10],
+        )?;
+        Ok(())
+    }
+
     pub fn upsert_appimage(
         &self,
         path: &str,
@@ -825,6 +854,7 @@ impl Database {
         release_history: Option<&str>,
         desktop_actions: Option<&str>,
         has_signature: bool,
+        screenshot_urls: Option<&str>,
     ) -> SqlResult<()> {
         self.conn.execute(
             "UPDATE appimages SET
@@ -843,7 +873,8 @@ impl Database {
                 project_group = ?14,
                 release_history = ?15,
                 desktop_actions = ?16,
-                has_signature = ?17
+                has_signature = ?17,
+                screenshot_urls = ?18
              WHERE id = ?1",
             params![
                 id,
@@ -863,6 +894,7 @@ impl Database {
                 release_history,
                 desktop_actions,
                 has_signature,
+                screenshot_urls,
             ],
         )?;
         Ok(())
@@ -907,7 +939,7 @@ impl Database {
          appstream_id, appstream_description, generic_name, license,
          homepage_url, bugtracker_url, donation_url, help_url, vcs_url,
          keywords, mime_types, content_rating, project_group,
-         release_history, desktop_actions, has_signature";
+         release_history, desktop_actions, has_signature, screenshot_urls";
 
     fn row_to_record(row: &rusqlite::Row) -> rusqlite::Result<AppImageRecord> {
         Ok(AppImageRecord {
@@ -964,6 +996,7 @@ impl Database {
             release_history: row.get(50).unwrap_or(None),
             desktop_actions: row.get(51).unwrap_or(None),
             has_signature: row.get::<_, bool>(52).unwrap_or(false),
+            screenshot_urls: row.get(53).unwrap_or(None),
         })
     }
 
@@ -1843,7 +1876,7 @@ mod tests {
             [],
             |row| row.get(0),
         ).unwrap();
-        assert_eq!(version, 9);
+        assert_eq!(version, 10);
 
         // All tables that should exist after the full v1-v7 migration chain
         let expected_tables = [