feat(market): per-exchange 24h volume in the hero

The hero's "24H Vol" showed CoinGecko's cross-exchange aggregate even after the
chart/price went per-venue. Capture the per-exchange converted_volume.usd from
the tickers (previously discarded, alongside converted_last) and show the
SELECTED exchange's own 24h volume; fall back to the aggregate when unknown.
Market cap stays aggregate (it's coin-wide, not per-venue).

This is a big real difference — e.g. Ourbit ~$14.3K vs NonKYC ~$253 for DRGX/USDT
— so the header now reflects the venue you're actually looking at.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/data/exchange_info.h

@@ -20,6 +20,7 @@ struct ExchangePair {
     std::string tradeUrl;    ///< Link to the exchange pair page
     std::string identifier;  ///< CoinGecko exchange id (e.g. "ourbit", "nonkyc_io") — keys the candle adapter
     double      lastUsd = 0.0; ///< This venue's current price in USD (CoinGecko converted_last.usd); 0 if unknown
+    double      volumeUsd = 0.0; ///< This venue's 24h volume in USD (CoinGecko converted_volume.usd); 0 if unknown
 };
 
 /**