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/app.h

@@ -295,6 +295,9 @@ public:
     // Fetch the live exchange/pair list from CoinGecko once per session (venues are
     // near-static); populates state.market.exchanges. Safe to call every frame.
     void refreshExchanges();
+    // 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();
     
     /// @brief Per-category refresh intervals, adjusted by active tab
     using RefreshIntervals = services::NetworkRefreshService::Intervals;
@@ -567,6 +570,7 @@ private:
     bool balance_refresh_pending_ = false;    // UI asked for an immediate refresh
     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
     util::AsyncTaskManager async_tasks_;
     bool pending_antivirus_dialog_ = false;  // Show Windows Defender help dialog