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/zh-TW.json

@@ -1266,17 +1266,14 @@
         "dragDrop": {
             "unableToResolveRoot": "無法確定移動的目標路徑。",
             "moveUnsupported": "此項目不支援移動。",
-            "createFolderHint": "放開以建立新資料夾",
             "newFolderName": "新資料夾名稱",
-            "folderNameHint": "按 Enter 確認，Escape 取消",
             "emptyFolderName": "請輸入資料夾名稱",
             "invalidFolderName": "資料夾名稱包含無效字元",
             "noDragState": "未找到待處理的拖放操作"
         },
         "empty": {
             "noFolders": "未找到資料夾",
-            "dragHint": "將項目拖到此處以建立資料夾",
-            "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": "檢查此資料夾的更新",