feat(market): card/chart refinements — 8-cell min width, larger labels, flat fill, range-matched change

Portfolio group cards:
- Always show the secondary DRGX amount, including on 2-cell cards.
- Larger group icon + label (subtitle1); value stays subtitle1 for consistency.
- Minimum group width is now 8 cells (default width 8).

Price chart:
- Area fill is now a flat translucent fill matching the group-card sparklines
  (dropped the vertical gradient + its primitive-API helper).
- Interval buttons sit near the top of the chart area (removed the empty gap
  above them) with a comfortable gap below before the plot.
- The change badge's suffix now matches the SELECTED range (1H->1h, 1D->24h,
  1W->7d, 1M->30d) instead of the raw data span (which read 23h / 6d).

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

src/config/settings.h

@@ -96,7 +96,7 @@ public:
         // (-1 = auto-place), w/h in cells (span). Set when the user drags/resizes a card.
         int gridCol = -1;
         int gridRow = -1;
-        int gridW = 6;
+        int gridW = 8;                 // default group width (cells); minimum is 8
         int gridH = 3;                 // default group height (cells); minimum is 2, card design adapts
     };