feat: add greet function (shmuel max)

[ShmuelMax100]

Summary

Adds a greet() function that returns a personalised welcome message.
This is the foundation for the onboarding banner we'll add in the next sprint.

Changes

• src/app.py — new greet(name) function with input validation
• src/test_app.py — two tests covering the happy path and empty-name edge case

Type of Change

• Bug fix
• [x ] New feature
• Refactor / cleanup
• CI / tooling
• Documentation

Testing

Ran the full test suite locally — all 10 tests pass:

cd src && pytest -v
# test_greet PASSED
# test_greet_empty_name PASSED

## Checklist

- [x ] My code follows the project's style guidelines
- [ x] I have added or updated tests where applicable
- [ x] CI checks are passing
- [ x] I have self-reviewed this PR before requesting review
- [ x] Related issues are linked below

## Related Issues

Closes #