Default to Catalog tab when library is empty on first launch

When no AppImages are found in the database, switch the ViewStack
to the Catalog page so users see the discovery experience first.

src/window.rs

@@ -1009,8 +1009,11 @@ impl DriftwoodWindow {
                 library_view.populate(records);
             }
             _ => {
-                // Empty database - show empty state
+                // Empty database - show empty state and default to Catalog tab
                 library_view.set_state(LibraryState::Empty);
+                if let Some(view_stack) = self.imp().view_stack.get() {
+                    view_stack.set_visible_child_name("catalog");
+                }
             }
         }