feat(market): candlestick chart for per-exchange OHLC

The exchange candle APIs return full OHLC but we only kept the close (a line).
Now the per-exchange chart draws real candlesticks; the CoinGecko aggregate stays
a line (it's close-only).

- Adapter keeps OHLC: parseExchangeOHLC() returns open/high/low/close candles
  (parseExchangeCandles is now a close-only wrapper over it). New data/candle.h
  holds the dependency-free Candle + bucketOHLC (5-min -> hourly for the 1D view).
- Model stores exchange_ohlc_intraday/daily alongside the close series;
  refreshExchangeChart populates both. market_series::chartCandles() returns the
  bucketed OHLC for the range, empty unless the per-exchange series is active.
- The chart renders wick (low..high) + body (open..close), green up / red down,
  with a low..high y-range; the line-only bits (fill, hi/lo labels, hover tooltip)
  are gated off for candles. Falls back to the line for the aggregate / Live view.

Verified: OHLC parsers + bucketOHLC + chartCandles unit-tested; a forced-state
render shows correct candlesticks; the aggregate still draws a clean line.

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

src/data/wallet_state.h

@@ -12,6 +12,7 @@
 #include <utility>
 
 #include "exchange_info.h"
+#include "candle.h"
 
 namespace dragonx {
 
@@ -182,6 +183,10 @@ struct MarketInfo {
     std::vector<std::pair<std::time_t, double>> exchange_chart_intraday;
     std::vector<std::pair<std::time_t, double>> exchange_chart_daily;
     bool exchange_chart_active = false;
+    // Full OHLC for the same per-exchange candles, backing the candlestick rendering (the aggregate
+    // CoinGecko series is close-only, so it stays a line). Parallel to exchange_chart_* above.
+    std::vector<data::Candle> exchange_ohlc_intraday;
+    std::vector<data::Candle> exchange_ohlc_daily;
 
     // Exchanges/pairs fetched live from CoinGecko (empty until fetched; the Market tab
     // falls back to data::getExchangeRegistry() while empty).