Add app icons, screenshots, and complete AppStream metainfo

- Add GNOME HIG-compliant app icon (scalable SVG) and symbolic variant
- Add 12 screenshots covering all major views and features
- Flesh out metainfo with screenshots, categories, URLs, content
  rating, system requirements, provides, translation, donation
  and contact links
- Update AppImage build script to bundle GTK plugin, symbolic
  icon, and metainfo
- Update meson.build with icon installation rules
- Remove About dialog from application menu
- Remove unused user guide and audit tool

meson.build

@@ -2,7 +2,7 @@ project(
   'driftwood',
   'rust',
   version: '0.1.0',
-  license: 'GPL-3.0-or-later',
+  license: 'CC0-1.0',
   meson_version: '>= 0.62.0',
 )
 
@@ -28,12 +28,25 @@ install_data(
   install_dir: datadir / 'metainfo',
 )
 
+# Install app icons
+install_data(
+  'data' / 'icons' / 'hicolor' / 'scalable' / 'apps' / app_id + '.svg',
+  install_dir: iconsdir / 'hicolor' / 'scalable' / 'apps',
+)
+install_data(
+  'data' / 'icons' / 'hicolor' / 'symbolic' / 'apps' / app_id + '-symbolic.svg',
+  install_dir: iconsdir / 'hicolor' / 'symbolic' / 'apps',
+)
+
 # Compile and install GSettings schema
 install_data(
   'data' / app_id + '.gschema.xml',
   install_dir: datadir / 'glib-2.0' / 'schemas',
 )
-gnome.post_install(glib_compile_schemas: true)
+gnome.post_install(
+  glib_compile_schemas: true,
+  gtk_update_icon_cache: true,
+)
 
 # Build the Rust binary via Cargo
 cargo = find_program('cargo')