feat(market): per-exchange price chart from each venue's own candle API

The market chart was hardwired to CoinGecko's cross-exchange USD aggregate
(vs_currency=usd), so selecting a trading pair only changed the "Trade" link —
never the chart or price. Now the SELECTED exchange drives both.

CoinGecko gives us which venues list DRGX (the ticker `market.identifier`) but
not their APIs, so add data/exchange_candles.h: a hand-maintained map from that
identifier to each venue's public candle endpoint, with two adapters verified
against the live APIs — Ourbit (MEXC-style /api/v3/klines, array format) and
NonKYC (TradingView-UDF /market/candles, `bars`). Unmapped venues return no URL,
so the chart falls back to the CoinGecko aggregate and nothing regresses.

- App::refreshExchangeChart() resolves the selected pair, fetches its intraday
  (5-min) + daily candles via the TLS-verified httpGetString on the worker, and
  stores them on MarketInfo (exchange_chart_intraday/daily + exchange_chart_active).
  Re-fetches on pair change; self-throttled to ~30 min otherwise; falls back to
  aggregate on any failure.
- chartSeries() draws the per-exchange series when active (portfolio sparklines
  stay on the aggregate). The hero shows the selected venue's real price
  (converted_last) — Ourbit vs NonKYC differ ~7%. parseCoinGeckoTickers now
  captures the identifier + per-exchange USD price (previously discarded).

Adapters unit-tested against real captured responses (URL builder + both parsers
+ the chartSeries switch) and validated against the full live feeds (Ourbit 20d,
NonKYC 370d, ascending, correct closes). Build + ctest + hygiene green.

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

src/app.h

@@ -335,7 +335,11 @@ public:
     // Fetch historical USD price series (CoinGecko market_chart) that back the portfolio
     // sparkline intervals; self-throttled to ~30 min. Safe to call every frame.
     void refreshMarketChart();
-    
+    // Fetch the SELECTED pair's candles from that exchange's own API (data/exchange_candles.h) so the
+    // Market chart shows the real per-exchange price. Re-fetches on pair change; falls back to the
+    // CoinGecko aggregate for unmapped venues / failed fetches. Safe to call every frame.
+    void refreshExchangeChart();
+
     /// @brief Per-category refresh intervals, adjusted by active tab
     using RefreshIntervals = services::NetworkRefreshService::Intervals;
 
@@ -675,6 +679,11 @@ private:
     bool balance_snapshot_seen_ = false;      // the current in-flight snapshot was applied
     bool exchanges_fetch_started_ = false;    // once-per-session CoinGecko tickers fetch
     bool chart_fetch_in_flight_ = false;      // a market_chart history fetch is on the worker
+    // Per-exchange candle chart (refreshExchangeChart): which pair the loaded series is for, an
+    // in-flight guard, and a slow refresh timer (candles move slowly, like the aggregate chart).
+    std::string exchange_chart_key_;          // "<identifier>:<BASE>/<QUOTE>" of the loaded series ("" = none)
+    bool exchange_chart_fetch_in_flight_ = false;
+    std::chrono::steady_clock::time_point exchange_chart_last_fetch_{};
     util::AsyncTaskManager async_tasks_;
     bool pending_antivirus_dialog_ = false;  // Show Windows Defender help dialog