feat: 实现文件夹和书签的持久排序与拖拽功能

openspec/changes/add-dnd-sorting/design.md

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+# Design: Persistent ordering + touch-friendly DnD
+
+## Database
+- Add `sort_order integer not null default 0` to `bookmarks`.
+- Add indexes to support ordered listing:
+  - `(user_id, folder_id, sort_order)`
+
+## API
+- Extend `Bookmark` DTO/schema with `sortOrder`.
+- Add `POST /bookmarks/reorder` similar to existing `/folders/reorder`:
+  - Input: `{ folderId: uuid|null, orderedIds: uuid[] }`
+  - Validates `orderedIds` is a permutation of all bookmarks for that user+folder (excluding deleted).
+  - Transactionally updates `sort_order` for each id.
+
+## Web UI
+- Replace native HTML5 drag/drop with a touch-capable approach.
+  - Implementation choice: `sortablejs` (small, proven, touch-friendly).
+  - Bind Sortable to:
+    - Folder header list (per parent group) for folder ordering.
+    - Each open folder’s bookmark list for bookmark ordering.
+- Root group is rendered as a first-class group and can also be reordered.
+
+## Compatibility
+- If the DB schema lacks ordering columns (fresh/old DB), endpoints should return a clear 409 prompting `db:migrate`.