Fix Rank Input Validation to Only Accept Natural Numbers

[sandeshlavshetty]

Solves issue #42

Description:
This pull request addresses the issue where the rank field in the form was accepting negative values, which should not be allowed since only natural numbers (positive integers) are valid inputs for rank.

Changes Made:

1. Added input validation to ensure that the rank field only accepts positive integers.
2. Implemented error handling to show an error message when the user enters an invalid rank (non-positive value).
3. Disabled the submit button if the rank is invalid or any required field is empty.
4. Updated the form to display an error message if the entered rank does not meet the validation criteria.

Steps to Re-create the Issue:

1. Navigate to the form and select an exam from the dropdown.
2. Enter a negative number or zero in the "Enter Category Rank" field.
3. Observe that the form accepts the input without showing any error message or restricting submission.

This fix ensures proper validation, preventing submission when the rank is not a natural number.

[suryabulusu]

looks good. please take a pull of recent changes before merging. i'll merge #39 first and then yours.