🎨 [README] Add live demo showcase with screenshots at top

[codeme-ne]

Problem

The README is well-structured but buries the lead. We have a working live demo at https://checkst-bot.vercel.app but recruiters/visitors won't see it until they scroll down.

Solution

Add a prominent demo section at the very top of the README:

1. Add Demo Banner

# checkstBot - RAG-Powered Learning Assistant

[![Live Demo](https://img.shields.io/badge/demo-live-brightgreen)](https://checkst-bot.vercel.app)
[![TypeScript](https://img.shields.io/badge/TypeScript-5.0-blue)](https://www.typescriptlang.org/)
[![Next.js](https://img.shields.io/badge/Next.js-14-black)](https://nextjs.org/)

> 🎓 **[Try the Live Demo →](https://checkst-bot.vercel.app)**

![checkstBot Demo](./docs/demo-screenshot.png)

2. Create Screenshots

• Take screenshot of main interface with document uploaded
• Take screenshot showing RAG chat in action
• Optional: Create a GIF showing the workflow (upload → chat → response)
• Store in /docs/ or /public/ folder

3. Add "Demo" Section After Features

## 🎬 Demo

See checkstBot in action:

| Upload Documents | Chat with AI | Get Contextual Answers |
|-----------------|--------------|----------------------|
| ![Upload](./docs/upload.png) | ![Chat](./docs/chat.png) | ![Answer](./docs/answer.png) |

Impact

HIGH - Visual demo is the #1 thing that makes recruiters stop and look. Currently they have to imagine what it looks like.

Effort

~15 minutes to take screenshots and update README

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Labels: enhancement, documentation, good first issue