refactor(sidebar): inline new-folder row, drop drag-to-blank creation (#999)

- Render the new-folder input as a temporary tree row at the creation
  location (file-explorer style): full-width input confirmed with Enter
  and canceled with Escape/blur; the parent folder auto-expands, and in
  list mode the row is inserted after the parent item
- Remove the drag-to-blank-area folder creation (drop-zone strip,
  sidebar-level drag handlers, performDragMoveWithState); dropping models
  onto folder nodes still moves them
- Update empty-state hints and locale keys accordingly

locales/es.json

@@ -1266,17 +1266,14 @@
         "dragDrop": {
             "unableToResolveRoot": "No se puede determinar la ruta de destino para el movimiento.",
             "moveUnsupported": "El movimiento no es compatible con este elemento.",
-            "createFolderHint": "Suelta para crear una nueva carpeta",
             "newFolderName": "Nombre de la nueva carpeta",
-            "folderNameHint": "Presiona Enter para confirmar, Escape para cancelar",
             "emptyFolderName": "Por favor, introduce un nombre de carpeta",
             "invalidFolderName": "El nombre de la carpeta contiene caracteres no válidos",
             "noDragState": "No se encontró ninguna operación de arrastre pendiente"
         },
         "empty": {
             "noFolders": "No se encontraron carpetas",
-            "dragHint": "Arrastra elementos aquí para crear carpetas",
-            "createHint": "[TODO: Translate] Click the New Folder button above, or drag items here to create folders"
+            "createHint": "[TODO: Translate] Click the New Folder button above to create folders"
         },
         "folderUpdateCheck": {
             "label": "Buscar actualizaciones en esta carpeta",