useExchangeRate hook returns a hardcoded mock price instead of a live rate

[abayomicornelius]

src/hooks/useExchangeRate.ts always resolves { xlmUsd: 0.12 } via setTimeout, regardless of real market conditions — there is no actual API call. This directly contradicts the README's "Live exchange quotes" feature claim and the CHANGELOG's "Exchange rate display" entry, meaning the price badge shown throughout the UI is permanently stale/wrong.

Wire this up to a real price feed (CoinGecko, backend /quotes endpoint, or Horizon-based DEX pricing) or clearly label it as a placeholder until implemented.