feat(market): real historical sparklines via CoinGecko market_chart

Portfolio-group sparklines previously resampled the ~24-minute in-session price
buffer, so the hour/day/week/month intervals could never fill. Fetch real
historical USD series from CoinGecko /market_chart and back each interval with
appropriately-grained data.

- MarketInfo gains two timestamped series: price_chart_intraday (days=1, 5-min)
  and price_chart_daily (days=365, daily), plus a fetch timestamp.
- App::refreshMarketChart() fetches both on the RPC worker via the TLS-verifying
  util::httpGetString helper, self-throttled to ~30 min (historical data moves
  slowly). Gated by getFetchPrices(); triggered on connect and each price tick.
- Pure NetworkRefreshService::parseCoinGeckoMarketChart() parses {"prices":
  [[ms,price],...]} into (unix-seconds, price); malformed rows skipped. Unit-tested.
- market_tab pfSparklineSeries() maps interval -> series+bucket: minute = live
  buffer; hour = intraday bucketed to 1h; day/week/month = daily bucketed to
  1d/7d/30d. Falls back to the in-session buffer until the fetch populates.

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

src/data/wallet_state.h

@@ -8,6 +8,8 @@
 #include <vector>
 #include <cstdint>
 #include <chrono>
+#include <ctime>
+#include <utility>
 
 #include "exchange_info.h"
 
@@ -159,9 +161,19 @@ struct MarketInfo {
     bool price_loading = false;
     std::string price_error;
     
-    // Price history for chart
+    // Live in-session price history: ~1 sample/minute, capped at MAX_HISTORY samples.
+    // Backs the main chart and the "minute" portfolio-sparkline interval.
     std::vector<double> price_history;
-    static constexpr int MAX_HISTORY = 24; // 24 hours
+    static constexpr int MAX_HISTORY = 24; // 24 samples (~24 minutes at the 60s refresh)
+
+    // Historical USD price series fetched from CoinGecko market_chart, so the portfolio-group
+    // sparklines can show real hour/day/week/month trends instead of resampling the ~24-minute
+    // in-session buffer. Timestamped (unix seconds), oldest->newest; empty until the first fetch.
+    // Refreshed on a slow cadence (~30 min) since historical data moves slowly.
+    std::vector<std::pair<std::time_t, double>> price_chart_intraday; // ~24h @ 5-minute granularity
+    std::vector<std::pair<std::time_t, double>> price_chart_daily;    // ~1yr @ daily granularity
+    std::chrono::steady_clock::time_point chart_last_fetch_time{};
+    bool chart_loaded = false;
 
     // Exchanges/pairs fetched live from CoinGecko (empty until fetched; the Market tab
     // falls back to data::getExchangeRegistry() while empty).