[FE]: Code challenge solution

[ScottQT]

feat(problem1): sum 1 to n with 3 implementations and tests

• Implement sum_to_n in three ways:
  A) Closed-form n(n+1)/2 — O(1) time and space.
  B) For-loop accumulation — O(n) time, O(1) space.
  C) Recursion with base case n<1 — O(n) time, O(n) stack.
• Edge cases: n<1 returns 0; non-integer input truncated via Math.floor.
• Add sum_to_n.test.js: normal cases (5, 1, 10, 100), n=0, negative n, non-integer.
• Export for Node; all implementations pure and deterministic.

---

feat(problem2): currency swap form with Vite, React, TypeScript

• Replace template with Vite + React; business logic in TypeScript, UI in JSX.
• Data: Switcheo prices.json and token-icons repo; dedupe by currency (latest date), omit tokens without price.
• Architecture: services (priceService), hooks (useTokensWithPrices, useSwapForm), utils (exchange, validation, tokenIcon, imagePreload), types (Token, PriceRecord).
• UI: SwapCard, TokenSelect, TokenIcon; validation, swap direction button, live exchange rate, amount to receive read-only.
• Preload token images on load for faster dropdown; fallback icon (first letter) when image fails.
• Toast on swap success/error with details (amount from/to) in English; auto-dismiss and close button.
• Run: cd src/problem2 && npm install && npm run dev

Forms UI

---

feat(problem3): WalletPage analysis and refactor

• ANALYSIS.md: document 14 issues — wrong filter variable (lhsPriority→balancePriority), inverted filter logic, missing blockchain on WalletBalance, unnecessary useMemo deps (prices), sort comparator missing return 0, formattedBalances unused and rows from wrong source, key={index}, getPriority recreated every render, any for blockchain, empty Props, logic in component, no error/loading handling, memoization — with fixes.
• Refactor: types (wallet.ts, Blockchain union), utils (walletBalance.ts — priority, filter, sort, format), hook (useSortedFormattedBalances), WalletPage UI-only with stable keys and no business logic.
• Aligns with separation of concerns, type safety, and performance practices.