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/appstream.rs

@@ -26,6 +26,7 @@ pub struct AppStreamMetadata {
     pub content_rating_summary: Option<String>,
     pub releases: Vec<ReleaseInfo>,
     pub mime_types: Vec<String>,
+    pub screenshot_urls: Vec<String>,
 }
 
 #[derive(Debug, Clone)]
@@ -64,6 +65,8 @@ pub fn parse_appstream_xml(xml: &str) -> Option<AppStreamMetadata> {
     let mut in_content_rating = false;
     let mut current_content_attr_id = String::new();
     let mut in_developer = false;
+    let mut in_screenshots = false;
+    let mut in_screenshot_image = false;
     let mut depth = 0u32;
     let mut description_depth = 0u32;
     let mut release_desc_depth = 0u32;
@@ -161,6 +164,14 @@ pub fn parse_appstream_xml(xml: &str) -> Option<AppStreamMetadata> {
                     "category" if in_categories => {
                         current_tag = "category".to_string();
                     }
+                    "screenshots" if in_component => {
+                        in_screenshots = true;
+                    }
+                    "image" if in_screenshots => {
+                        // Prefer "source" type, but accept any <image>
+                        in_screenshot_image = true;
+                        current_tag = "screenshot_image".to_string();
+                    }
                     "developer" if in_component => {
                         in_developer = true;
                     }
@@ -252,6 +263,12 @@ pub fn parse_appstream_xml(xml: &str) -> Option<AppStreamMetadata> {
                     "categories" => {
                         in_categories = false;
                     }
+                    "screenshots" => {
+                        in_screenshots = false;
+                    }
+                    "image" if in_screenshot_image => {
+                        in_screenshot_image = false;
+                    }
                     "developer" => {
                         in_developer = false;
                     }
@@ -301,6 +318,11 @@ pub fn parse_appstream_xml(xml: &str) -> Option<AppStreamMetadata> {
                     "category" => {
                         meta.categories.push(text);
                     }
+                    "screenshot_image" => {
+                        if text.starts_with("http") && meta.screenshot_urls.len() < 10 {
+                            meta.screenshot_urls.push(text);
+                        }
+                    }
                     "developer_name" => {
                         if meta.developer.is_none() {
                             meta.developer = Some(text);