Add comprehensive AppImage metadata extraction, display, and bug fixes

- Add AppStream XML parser (quick-xml) to extract rich metadata from bundled
  metainfo/appdata files: description, developer, license, URLs, keywords,
  categories, content rating, release history, and MIME types
- Database migration v9: 16 new columns for extended metadata storage
- Extended inspector to parse AppStream XML, desktop entry extended fields,
  and detect binary signatures without executing AppImages
- Redesigned detail view overview tab with 8 conditional groups: About,
  Description, Links, Release History, Usage, Capabilities, File Info
- Fix crash on exit caused by stale GLib SourceId removal in debounce timers
- Fix wayland.rs executing AppImages directly to detect squashfs offset,
  replaced with safe binary scan via find_squashfs_offset_for()
- Fix scan skipping re-analysis of apps missing new metadata fields

src/core/mod.rs

@@ -1,4 +1,5 @@
 pub mod analysis;
+pub mod appstream;
 pub mod backup;
 pub mod database;
 pub mod discovery;