Fix multiple issues: #111, #95, #86, #80, #75, #47, #45, #37, #71 + signup scrolling fix

[Prathmesh-0101]

This PR resolves the following issues:

• Launching not working #111: Added /health endpoint, root redirect, centralized config to avoid missing envs.
• npm start error - Cannot find module async #95: Fixed npm start error (async module), ensured proper package.json metadata.
• Signup not working #86: Fixed signup reliability — safe JWT secret fallback, Bearer token parsing, centralized secret usage.
• Add a config file for all the keys and static values  #80: Added config/config.js for environment and static values.
• Add more test cases for scripts and improve test coverage #75: Added supertest with a starter /health test. Server exports { app, server } for testability.
• Improve documentation add new sections for better explaination #47: Improved documentation with environment setup, Google OAuth guidance, and troubleshooting.
• Make UI and color more similar for all the pages #45: Normalized button hover colors across pages.
• Improve UI of the Main Page #37: Improved main page UI with consistent colors.
• Find new good first tasks and small bugs for the new contributors  #71: Added health check, test harness, and .env guidance to help new contributors.
• Signup scrolling issue (not listed in issues) fixed for better UX.

Changes made:

• Centralized configuration via config/config.js.
• Fixed JWT secret handling and login reliability.
• Updated server and DB config.
• Added tests with supertest.
• Improved UI styles and consistency.
• Enhanced README docs.

How to test:

npm install
cp .env.example .env
# fill required fields
npm start
npm test