feat(ui): Add download transcript feature for chat sessions

[nancysangani]

🔗 Related Issue

Closes #426

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📝 What does this PR do?

Creates backend/app/routes/chat_export.py — the missing module already
imported by the /chat/export/{document_id}?format=pdf route in chat.py.

The frontend export UI (Download button, MD/TXT/PDF dropdown in ChatPanel.tsx)
and the backend export endpoint (GET /chat/export/{document_id}) were already
fully implemented. The only missing piece was format_pdf() which the endpoint
imports from app.routes.chat_export.

backend/app/routes/chat_export.py (new):

• format_pdf(doc, messages) — renders chat history as a clean A4 PDF using
  ReportLab (already in requirements.txt).
• Cover block: document name, export timestamp, message count, indigo rule.
• Per-message blocks: colour-coded role label (indigo = user, emerald =
  assistant), timestamp, body text with XML escaping and soft line-wrapping,
  source citations with filename / page / confidence / 120-char preview.
• Horizontal dividers between messages for readability.
• _safe_text() escapes &, <, >, " so ReportLab Paragraph never
  crashes on user-generated content.
• _wrap_body() soft-wraps long lines at 100 chars and converts \n to
  <br/> so multi-paragraph answers render correctly.

No changes to chat.py, ChatPanel.tsx, or any other existing file.

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🗂️ Type of Change

• ✨ New feature

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🧪 How was this tested?

• Ran the backend locally (uvicorn app.main:app --reload)
• Uploaded a document, had a multi-turn conversation, clicked Export → PDF
  — file downloaded and opened correctly in a PDF viewer
• Verified cover block shows correct document name, timestamp, and message
  count
• Verified user and assistant turns are colour-coded correctly
• Verified source citations render with filename, page, confidence, and
  preview text
• Verified messages containing &, <, > and multi-line content render
  without crashing
• Confirmed MD and TXT export paths are unaffected (no regression)

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

[nancysangani]

Hi @param20h, Please review the PR when you get a chance. Thanks!

[github-actions]

🎉 Congratulations on getting your Pull Request merged! 🎉

Thank you for contributing to PDF-Assistant-RAG as part of GSSoC '26! 🚀

Keep up the great work! ✨