feat: add Tip Calculator and Bill Splitter tool

[rayrishu19-wq]

This PR adds a new "Tip Calculator & Bill Splitter" tool to the Math category.
It allows users to calculate gratuity and split bills among multiple people with a clean, responsive UI.

Closes #1778

Type of Change

• New Feature

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have tested the logic locally

[gornvan]

How is it an IT tool?

[rayrishu19-wq]

@gornvan Fair question! While it's not a 'technical' tool like a JWT debugger or CRON parser, I added it to the Math category as a general productivity utility.

Developers and IT teams often work in office environments where splitting bills for team lunches or calculating gratuity for events is a frequent, minor task. Having a clean, ad-free, and responsive tool for this within a familiar 'Swiss Army knife' interface like it-tools is quite convenient. Many similar developer boxes (like DevToys) include basic math and productivity utilities for exactly this reason."

[sonarqubecloud]

Quality Gate failed

Failed conditions
15.7% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud