admin: restore nested mode (with subject _index.md titles so tree shows named folders)

app/src/web/admin/config.yml

@@ -29,6 +29,11 @@ collections:
     folder: subjects
     create: true
     slug: "{{slug}}"
+    # nested mode: shows subjects/<subject>/<slug>/ tree (Decap-native for subfolders)
+    nested:
+      depth: 2
+    meta:
+      path: { widget: string, label: Path, index_file: _index, editable_folder: false, searchable: false }
     sortable_fields: [date, title]
     fields:
       - { name: title, label: Title, widget: string }