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

[adikulkarni006]

Problem
Sessions are listed in sidebar but cannot be reordered manually. Users may want to pin or reorder important conversations.

Expected behavior

• Drag any session up/down in the sidebar list.
• Visual placeholder/drop indicator during drag.
• After drop, new order persists via API call.
• Backend: Add PATCH /api/sessions/reorder endpoint to update order_index field.

Technical hints

• Frontend: Use @dnd-kit/sortable or react-beautiful-dnd
• Add order field to session table (SQLite)
• Update services/db_service.py to support batch update of order indices
• Ensure it works with existing session search/filter

Labels: enhancement, good-first-issue, SSoC26