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/services/network_refresh_service.h

@@ -250,6 +250,10 @@ public:
                                                                          std::time_t fetchedAt);
     static PriceHttpResult parsePriceHttpResponse(const PriceHttpResponse& response,
                                                   std::time_t fetchedAt);
+    // Parse a CoinGecko /market_chart response ({"prices":[[ms,price],...]}) into a timestamped
+    // (unix seconds, USD price) series, oldest->newest. Malformed rows are skipped; on any error
+    // (or a missing "prices" array) an empty vector is returned. Pure/no-I/O.
+    static std::vector<std::pair<std::time_t, double>> parseCoinGeckoMarketChart(const std::string& response);
     static AddressInfo buildShieldedAddressInfo(const std::string& address,
                                                 const nlohmann::json& validation,
                                                 bool validationSucceeded);