top k parameter Chat API

[soniyaLama0]

## 📋 PR Checklist

Thank you for contributing to PDF-Assistant-RAG! 🎉
Please fill out this template before submitting. PRs without it filled in will be closed.

---

🔗 Related Issue

N/A

📝 What does this PR do?

This PR adds a configurable top-k retrieval parameter to the Chat API, allowing the user to dynamically adjust the number of context chunks retrieved during RAG queries.

Specifically, it:

1. Adds a top_k parameter to the ChatRequest schema.
2. Updates /ask and /ask/stream endpoints to pass the top_k value down to the retriever.
3. Adds a "Sources count (Top-K)" slider in the Frontend Chat Settings menu to let users easily configure this value before asking questions.

---

🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

---

🧪 How was this tested?

• Ran the backend locally (uvicorn app.main:app --reload)
• Ran the frontend locally (npm run dev inside frontend/)
• Tested the affected API endpoints manually
• Added / updated tests

---

📸 Screenshots (if UI change)

---

⚠️ Anything to flag for reviewers?

The default fallback for top_k remains unchanged (driven by backend settings) if the frontend does not explicitly provide a value.

---

✅ 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

[param20h]

mergeeee conflicts