[FEATURE] Add drag-and-drop reordering of chat sessions in sidebar

[adikulkarni006]

Problem
Sessions in sidebar are currently displayed in a fixed order (e.g., by creation date). Users cannot manually reorder important sessions to the top.

Expected behavior

• Drag any session up/down in the sidebar list.
• Visual placeholder/drop indicator during drag.
• After drop, the new order persists across page reloads.
• Backend: Add order_index field to sessions table and a PATCH /api/sessions/reorder endpoint.

Technical hints

• Frontend: frontend/src/components/Sidebar.jsx – use @dnd-kit/sortable or react-beautiful-dnd.
• Backend: backend/routes/sessions.py – new endpoint, update db_service to support batch update of order indices.

Labels: enhancement, good-first-issue, SSoC26