feat(todo): add drag-and-drop reordering for checklist items

Add order_index column to checklist_items table to persist custom ordering.
Implement DnD-kit for drag-and-drop UI with vertical axis restriction.
Separate incomplete/completed items visually and only allow reordering within same completion state.
Update toggle logic to assign order index when moving between states.

Author: furkanksl

bun.lockb

@@ -18,6 +18,7 @@
     "@ai-sdk/mistral": "latest",
     "@ai-sdk/openai": "latest",
     "@dnd-kit/core": "^6.3.1",
+    "@dnd-kit/modifiers": "^9.0.0",
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
     "@hookform/resolvers": "^5.2.2",