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.h

@@ -89,6 +89,7 @@ public:
         bool showDrgx = true;           // show the DRGX amount on the card
         bool showValue = true;          // show the converted/fiat value on the card
         bool show24h = false;           // show the 24h % change (live-market bases only)
+        bool showSparkline = false;     // show a price-trend sparkline (live-market bases only)
     };
 
     // Theme