MVP 1: Build frontend and API for latest data

[Boxuan-Matty-Lin]

Goal

The frontend fetches from /api/rates and displays an AUD → USD/EUR/JPY/GBP/CNY conversion list.

Tasks

• Create GET /api/rates to proxy OXR latest.json (cache: "no-store").
• Add .env.dev with:
  - OXR_APP_ID
  - OXR_BASE_URL=https://openexchangerates.org/api
• Home page: amount input + 5 result rows (AUD → target).
• Loading/error states and a Refresh rates button.

Acceptance

• Page fetches data and shows 5 currencies.
• Changing the amount updates results instantly.
• On failure, show an error and allow refresh to recover.