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/candle.h

@@ -0,0 +1,44 @@
+#pragma once
+// A single OHLC candle — the shared shape for per-exchange candlestick charts. Kept in its own tiny,
+// dependency-free header so both the parser (data/exchange_candles.h, which pulls in nlohmann/json)
+// and the model (data/wallet_state.h, included widely) can use it without dragging json everywhere.
+#include <ctime>
+#include <vector>
+
+namespace dragonx {
+namespace data {
+
+struct Candle {
+    std::time_t time = 0;   // epoch SECONDS (the candle's open/bucket time)
+    double open  = 0.0;
+    double high  = 0.0;
+    double low   = 0.0;
+    double close = 0.0;
+};
+
+// Aggregate fine candles into fixed time buckets (e.g. 5-min -> hourly for the 1D view): open = first
+// open, high = max high, low = min low, close = last close. Input must be ascending by time.
+inline std::vector<Candle> bucketOHLC(const std::vector<Candle>& src, long windowSec) {
+    std::vector<Candle> out;
+    if (src.empty() || windowSec <= 0) return out;
+    long curBucket = -1;
+    Candle cur;
+    for (const auto& c : src) {
+        const long b = (long)(c.time / windowSec);
+        if (b != curBucket) {
+            if (curBucket >= 0) out.push_back(cur);
+            curBucket = b;
+            cur = c;
+            cur.time = (std::time_t)(b * windowSec);
+        } else {
+            if (c.high > cur.high) cur.high = c.high;
+            if (c.low  < cur.low)  cur.low  = c.low;
+            cur.close = c.close;
+        }
+    }
+    if (curBucket >= 0) out.push_back(cur);
+    return out;
+}
+
+} // namespace data
+} // namespace dragonx