feat(market): optional price-trend sparkline per portfolio group

Add a per-group "Sparkline" toggle (live-market bases only, like 24h). When on,
the group card draws a faint price-trend line in its lower band — the group's
value tracks the DRGX price, so it plots the market price_history (normalized),
colored green/red by overall direction. Behind the text so amounts stay
readable. The editor's live preview reflects it too.

Persisted as show_sparkline in settings.json; wired through PortfolioEntry +
the editor working state + the pfDrawSparkline() helper shared by card+preview.

Full-node + Lite build clean; ctest 1/1; hygiene clean. Rendering change —
needs a screenshot check.

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

src/config/settings.cpp

@@ -330,6 +330,8 @@ bool Settings::load(const std::string& path)
                     entry.showValue = e["show_value"].get<bool>();
                 if (e.contains("show_24h") && e["show_24h"].is_boolean())
                     entry.show24h = e["show_24h"].get<bool>();
+                if (e.contains("show_sparkline") && e["show_sparkline"].is_boolean())
+                    entry.showSparkline = e["show_sparkline"].get<bool>();
                 if (!entry.label.empty()) portfolio_entries_.push_back(std::move(entry));
             }
         }
@@ -487,6 +489,7 @@ bool Settings::save(const std::string& path)
         entry["show_drgx"] = e.showDrgx;
         entry["show_value"] = e.showValue;
         entry["show_24h"] = e.show24h;
+        entry["show_sparkline"] = e.showSparkline;
         j["portfolio_entries"].push_back(std::move(entry));
     }
     j["font_scale"]     = font_scale_;