feat(market): auto-populate exchange pairs from CoinGecko (adds OurBit); DRGX dashboard

- Fetch /coins/dragonx-2/tickers once per session (App::refreshExchanges, reusing the
  price-fetch worker path) and build the exchange registry at runtime into
  state.market.exchanges; parseCoinGeckoTickers groups tickers by venue. The Market tab
  sources from this live list and falls back to the compiled-in registry (now including
  OurBit + Nonkyc.io) when offline. Fixes the missing OurBit pair and auto-tracks future
  listings.
- Fix stale selection defaults (TradeOgre / DRGX-BTC -> Nonkyc.io / DRGX/USDT) that never
  matched the registry, and make the attribution generic ("Price data from CoinGecko").
- Reframe the single-asset portfolio card as a DRGX holdings dashboard: relabel to
  "MY DRGX" and show the 24h change on the holdings value (colored by direction).

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

src/data/wallet_state.h

@@ -9,6 +9,8 @@
 #include <cstdint>
 #include <chrono>
 
+#include "exchange_info.h"
+
 namespace dragonx {
 
 /**
@@ -160,6 +162,10 @@ struct MarketInfo {
     // Price history for chart
     std::vector<double> price_history;
     static constexpr int MAX_HISTORY = 24; // 24 hours
+
+    // Exchanges/pairs fetched live from CoinGecko (empty until fetched; the Market tab
+    // falls back to data::getExchangeRegistry() while empty).
+    std::vector<data::ExchangeInfo> exchanges;
 };
 
 /**