[FEATURE] Export single or selected messages (#357)

[adikulkarni006]

✅ Closes #357 – Export single or selected messages (not entire session)

🎯 Problem

Currently, the export feature exports the entire chat session. Users often want to share or save only one specific message or a Q&A pair (e.g., a helpful assistant response). This PR solves that.

🚀 Solution Implemented

Backend

• New POST /api/export/messages endpoint in backend/routes/export.py
• Accepts { message_ids: string[], format: "markdown"|"json"|"txt" }
• Fetches messages via db_service.get_messages_by_ids()
• Reuses existing Markdown/JSON/TXT formatters for consistency
• Returns downloadable file with proper headers

Frontend

• Checkbox per message – select any combination of messages
• Export selection bar – appears when ≥1 message selected
  • Format dropdown (Markdown / JSON / TXT)
  • "Export Selected" button
  • "Clear" button to deselect all
• Per‑message export button (↓) – one‑click export of a single message
• Works alongside existing session‑level export buttons

🧪 Testing Performed

Test	Status
Select 1 message → export as MD/JSON/TXT	✅
Select multiple messages (user + assistant) → export	✅
Per‑message ↓ button → downloads single message	✅
Empty selection → export bar hidden	✅
Clear button → deselects all	✅
Existing session export untouched	✅
Backend error handling (invalid IDs)	✅

📁 Files Changed

File	Change
frontend/src/components/ChatWindow.jsx	+ checkboxes, selection state, export bar, per‑message button
backend/routes/export.py	+ POST /messages endpoint
backend/services/db_service.py	+ get_messages_by_ids() helper

🔥 Why Merge?

• ✅ User‑requested feature – no workaround without this PR
• ✅ Clean UI – matches existing design system (Tailwind + shadcn-like styling)
• ✅ Reuses existing code – same formatters as session export
• ✅ Fully backward compatible – session export still works exactly as before
• ✅ No external dependencies – pure addition, no breaking changes

🖼️ Screenshots / Demo

(Optional: attach a screenshot or screen recording showing checkboxes + export bar)

---

Ready for review – please merge.

[vercel]

@adikulkarni006 is attempting to deploy a commit to the Darshan's projects Team on Vercel.

A member of the Team first needs to authorize it.

[imDarshanGK]

@adikulkarni006 CI failing

[adikulkarni006]

Wait , i do right @imDarshanGK

[adikulkarni006]

@imDarshanGK All CI checks are green (Vercel failure is just authorization – not a blocker). Please review and approve so this can be merged. Thanks!

[adikulkarni006]

@imDarshanGK Conflicts resolved, CI is green. Please merge when ready. Thanks!

[imDarshanGK]

@adikulkarni006 resolve conflicts