feat: implemented unit converter

[Satvik-Singh192]

Description

This pull request adds a fully functional Unit Converter feature as requested in Issue #30.
The implementation is delivered in a single, self-contained C++ file (unit_converter.cpp) to avoid excessive file splitting while maintaining modular structure through functions and namespaces.

Key Features

• Console-based, menu-driven interface
• Supports conversions for:
  • Length (meters, kilometers, miles, feet)
  • Temperature (Celsius, Fahrenheit, Kelvin)
  • Weight (grams, kilograms, pounds)
  • Volume (liters, milliliters, gallons)
• Input validation for incorrect numeric entries
• Clean helper utilities for safe input and formatting
• Extendable structure for adding new unit groups easily

This feature delivers a simple, fast, and accurate unit conversion tool directly through the console, fulfilling all requirements stated in the issue.

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Semver Changes

• Minor (new features, no breaking changes)

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Issues

This PR closes:

• [Feature]: Unit Converter #30 — [Feature]: Unit Converter

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Checklist

• I have read the Contributing Guidelines.
• Feature implemented according to the requirements.
• Code compiles and runs correctly.
• Input validation added.
• Modular structure kept in a single file as requested.
• Verified example conversions manually.

[Satvik-Singh192]

@prajwal3114 please review my PR sir