feat: mark fully checked collapsed diff folders

src/files.rs

@@ -370,6 +370,20 @@ impl FilePicker {
         })
     }
 
+    /// Full catalog descendants, independent of filtering and folder expansion.
+    pub(crate) fn descendant_catalog_indices<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> impl Iterator<Item = usize> + 'a {
+        let start = self
+            .nodes
+            .partition_point(|node| node.path.as_path() <= path);
+        self.nodes[start..]
+            .iter()
+            .take_while(move |node| node.path.starts_with(path))
+            .filter_map(|node| node.catalog_index)
+    }
+
     fn selected_node(&self) -> Option<usize> {
         self.rows.get(self.selected).map(|row| row.node)
     }