fix(ui): check icon theme exists on disk before fallback

has_icon() returns true even for broken themes since it checks all
search paths. Instead, verify the theme directory actually exists
in the search path. Falls back to Adwaita only when the configured
theme is genuinely missing from disk.

crates/owlry/src/app.rs

@@ -188,13 +188,19 @@ impl OwlryApp {
         if let Some(display) = gtk4::gdk::Display::default() {
             let icon_theme = gtk4::IconTheme::for_display(&display);
 
-            // If the system icon theme can't resolve standard icons (e.g., the
-            // configured theme doesn't exist on disk), fall back to Adwaita
-            // which is guaranteed to be installed as a GTK4 dependency.
-            if !icon_theme.has_icon("edit-find-symbolic") {
-                debug!(
-                    "System icon theme '{}' cannot resolve standard icons, falling back to Adwaita",
-                    icon_theme.theme_name()
+            // If the system icon theme doesn't exist on disk (e.g., set in
+            // gsettings but not installed), GTK falls back to hicolor which
+            // has almost no icons. Detect this and use Adwaita instead.
+            let theme_name = icon_theme.theme_name();
+            let theme_exists = icon_theme
+                .search_path()
+                .iter()
+                .any(|p| p.join(theme_name.as_str()).is_dir());
+
+            if !theme_exists && theme_name != "hicolor" && theme_name != "Adwaita" {
+                info!(
+                    "Icon theme '{}' not found on disk, falling back to Adwaita",
+                    theme_name
                 );
                 icon_theme.set_theme_name(Some("Adwaita"));
             }