feat(auth): add demo user seeding for local development

[Ayush9791]

Description

This PR improves the local development and contributor onboarding experience by adding automatic demo user seeding and updating the backend setup flow

##Type of Change

• [✅] Bug fix
• [✅] New feature
• Documentation update
• Refactoring

Changes

• Added "seedDemoUser.js" utility to create demo user automatically if it does not already exist
• Prevented repeated demo user creation by checking existing records
• Improved local authentication setup for fresh MongoDB instances

Problem Solved

Previously, the frontend displayed demo login credentials, but fresh local database setups does not contain the demo user, causing login attempts to fail with "401 Unauthorized".

This PR ensures contributors can run the project locally without manually inserting demo authentication data.

Checklist

• [✅] Code follows project style
• [✅] Tested locally
• Updated documentation

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