DOCS UPDATE:Local Setup & Flutter Version Guidance

### Is this related to an existing part of the documentation?

- [x] Yes, it is related to an existing section

### What needs to be updated?

Problem -
While attempting to reproduce and work on issues locally, the setup process was unclear and led to multiple environment/configuration problems.

Observed Issues -

No documented Flutter SDK version requirement
.env setup step is not clearly documented
flutter run -d chrome on main currently results in multiple compilation issues
Contributors may face dependency/version conflicts during setup
Suggested Improvements
Add recommended Flutter SDK version to README
Document .env.example -> .env setup step
Add a verified local setup workflow
Mention current web build limitations (if known)
Why This Helps

This would make onboarding smoother for new contributors and reduce setup/debugging friction before contributors can start working on actual issues.

I would like to fix this documentation issue.
Kindly assign it to me under GSSOC'26

### Relevant Documentation Link (if any)

_No response_

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this update