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

src/application.rs

@@ -3,7 +3,7 @@ use adw::subclass::prelude::*;
 use gtk::gio;
 use std::cell::OnceCell;
 
-use crate::config::{APP_ID, VERSION};
+use crate::config::APP_ID;
 use crate::window::DriftwoodWindow;
 
 mod imp {
@@ -118,28 +118,8 @@ impl DriftwoodApplication {
             })
             .build();
 
-        // About action
-        let about_action = gio::ActionEntry::builder("about")
-            .activate(|app: &Self, _, _| {
-                app.show_about_dialog();
-            })
-            .build();
-
-        self.add_action_entries([quit_action, about_action]);
+        self.add_action_entries([quit_action]);
         self.set_accels_for_action("app.quit", &["<Control>q"]);
     }
 
-    fn show_about_dialog(&self) {
-        let dialog = adw::AboutDialog::builder()
-            .application_name("Driftwood")
-            .application_icon(APP_ID)
-            .version(VERSION)
-            .developer_name("Driftwood Contributors")
-            .license_type(gtk::License::Gpl30)
-            .comments("A modern AppImage manager for GNOME desktops")
-            .website("https://github.com/driftwood-app/driftwood")
-            .build();
-
-        dialog.present(self.active_window().as_ref());
-    }
 }

src/window.rs

@@ -184,7 +184,6 @@ impl DriftwoodWindow {
         let section3 = gio::Menu::new();
         section3.append(Some(&i18n("Preferences")), Some("win.preferences"));
         section3.append(Some(&i18n("Keyboard Shortcuts")), Some("win.show-shortcuts"));
-        section3.append(Some(&i18n("About Driftwood")), Some("app.about"));
         menu.append_section(None, &section3);
 
         // Library view (contains header bar, search, grid/list, empty state)