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/config/settings.h

@@ -437,8 +437,8 @@ private:
     bool theme_effects_enabled_ = true;
     bool low_spec_mode_ = false;
     bool reduce_motion_ = false;
-    std::string selected_exchange_ = "TradeOgre";
-    std::string selected_pair_ = "DRGX/BTC";
+    std::string selected_exchange_ = "Nonkyc.io";
+    std::string selected_pair_ = "DRGX/USDT";
 
     // Pool mining
     std::string pool_url_       = "pool.dragonx.is:3433";