📸 Add screenshots, live demo section, and seed data

- 6 app screenshots (login, dashboard, kanban, card detail, users, email)
- Live demo URL with 4 demo accounts (admin + 3 members)
- Demo data: 3 boards, 21 cards, labels, comments, activity
- Seed script for reproducible demo environment
- Updated README with screenshots section and demo credentials

server/package.json

@@ -16,6 +16,7 @@
     "jsonwebtoken": "^9.0.2",
     "mailparser": "^3.7.2",
     "nodemailer": "^6.10.0",
+    "puppeteer-core": "^24.40.0",
     "uuid": "^11.0.5"
   }
 }