[FE]: Code challenge solution - Ngo Viet Anh

[vietanh1606]

This PR adds solutions for Problems 1, 2, and 3 of the code challenge.

Problem 1: Three Ways to Sum to N

• Added three JavaScript implementations of sum_to_n.
• Implemented using:
• iterative loop
• Gauss arithmetic formula
• array reduce
• Supports positive integers, zero, and negative integers.
• Added README with implementation notes, complexity, and quick test examples.

Problem 2: Fancy Form

• Built a currency swap form using Vite, React, and Tailwind CSS.
• Fetches live token prices from the Switcheo price API.
• Uses token icons from the Switcheo token icon repository.
• Added custom token selector with icon and price display.
• Calculates receive amount and exchange rate automatically.
• Added validation for invalid amount, same-token swap, and insufficient balance.
• Added mocked wallet balances and mocked swap submission loading state.
• Added responsive, clean UI with blue accent styling.
• Added README with run/build instructions and preview image placeholder.

Live Demo
https://fancy-form-steel.vercel.app/

Problem 3: Messy React

• Added analysis of computational inefficiencies and anti-patterns in the provided React/TypeScript snippet.
• Documented issues including:
• missing type fields
• incorrect filter logic
• undefined variables
• unstable sort comparator
• incorrect useMemo dependencies
• unused formatted data
• index-based React keys
• missing price fallback
• Added a refactored TypeScript/React version.
• Improved memoization by separating sorted balances from formatted balances.