fix: nav_context_menu must produce one submenu per nav item

The segmented_button widget asserts that context_menu children
count matches model length. The yoda fork was building a single
flat menu for the clicked item instead of iterating over all
nav_model items like upstream, causing an assertion panic on
right-click in the sidebar (trash, folders, etc.).

Align with upstream: iterate self.nav_model.iter() and use
menu::nav_context() instead of menu::items().

src/app.rs

@@ -2833,7 +2833,7 @@ impl Application for App {
     fn nav_context_menu(
         &self,
     ) -> Option<Vec<widget::menu::Tree<cosmic::Action<Self::Message>>>> {
-        let entity = self.nav_bar_context_id;
+        let items = self.nav_model.iter().map(|entity| {
             let favorite_index_opt = self.nav_model.data::<FavoriteIndex>(entity);
             let location_opt = self.nav_model.data::<Location>(entity);
 
@@ -2919,8 +2919,13 @@ impl Application for App {
                     NavMenuAction::EmptyTrash,
                 ));
             }
+            items
+        });
 
-        Some(cosmic::widget::menu::items(&HashMap::new(), items))
+        Some(cosmic::widget::menu::nav_context(
+            &HashMap::new(),
+            items.collect(),
+        ))
     }
 
     fn nav_model(&self) -> Option<&segmented_button::SingleSelectModel> {