[FEATURE] Drag-and-drop reordering of chat sessions in sidebar (#361)

[adikulkarni006]

Closes #361

🎯 Problem

Sessions in sidebar are displayed in a fixed order (by creation or update date). Users cannot manually reorder important sessions to the top.

🚀 Solution Implemented

Adds drag‑and‑drop reordering to the session list:

• Backend

  • Added order_index column to sessions table (default 0).
  • get_all_sessions() now orders by order_index ASC, updated_at DESC.
  • New update_sessions_order(session_ids) function to batch‑update indices.
  • New endpoint PATCH /api/sessions/reorder accepting a list of session IDs in the desired order.

• Frontend

  • Installed @dnd-kit/core, @dnd-kit/sortable, @dnd-kit/utilities.
  • Replaced static session list in Sidebar.jsx with a sortable DndContext.
  • Each session is a useSortable item – drag handle is the whole row (click‑to‑load still works).
  • Visual placeholder while dragging, opacity feedback.
  • After drop, new order is sent to backend via reorderSessions() API and sessions list is refreshed.
  • Added refreshSessions prop in App.jsx to update list after reorder.

• API

  • Added reorderSessions(sessionIds) function in api.js.

🧪 Testing Performed

• ✅ Drag any session up/down – order changes instantly.
• ✅ After drop, "Saving order..." message appears briefly.
• ✅ Page reload – order persists (backed by order_index).
• ✅ Existing functionality (new chat, delete, load, rename) remains unaffected.
• ✅ Works with search filter – only visible sessions are draggable, but reorder applies to full list (consistent with expectations).

📁 Files Changed

File	Change
backend/services/db_service.py	+ ensure_order_index_column(), modified get_all_sessions(), + update_sessions_order()
backend/routes/sessions.py	+ ReorderSessionsRequest model, + PATCH /reorder endpoint
frontend/src/components/Sidebar.jsx	Full rewrite with @dnd-kit sortable list
frontend/src/utils/api.js	+ reorderSessions() export
frontend/src/App.jsx	Pass refreshSessions to Sidebar
package.json (frontend)	Added @dnd-kit dependencies

🔥 Why Merge?

• ✅ Real user need – manual session reordering is a common request.
• ✅ Clean implementation – uses modern dnd-kit (lightweight, accessible).
• ✅ Persistent – order survives page reloads and server restarts.
• ✅ Backwards compatible – existing sessions get order_index = 0 and keep update‑based fallback order.
• ✅ No breaking changes – drag is an addition, no existing functionality removed.

Ready for review.

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[adikulkarni006]

@imDarshanGK Branch updated. All checks green. Please merge. Thanks!