Fix eval() security risk, input validation, and symbol typos in Converter

[Prayas340]

Description

A short summary of what is included in your Pull Request.

Refactored Converter/converter.py to eliminate eval(), introduced safe dictionary lookups via CATEGORIES, added input validation for unit symbols/categories/values, and corrected symbol typos in Converter/converter_values.py.

• Issue_no:
• Not applicable

Type of change

Choosing one or more options from the following as per the nature of your Pull request.

• New project insert
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation Update

Project

• Your Project Name: Converter

• Short Description: A Python command-line utility for converting length, area, volume, mass, and time units.

Checklist:

• I have named my files and folder, according to this project's guidelines.
• My code follows the style guidelines of this project.
• I have commented on my code, particularly in hard-to-understand areas.
• I have created a helpful and easy to understand README.md, according to the given README_TEMPLATE.
• My changes do not produce any warnings.