feat(market): fine 32px square portfolio grid that scales to width and scrolls

Replace the fill-to-height portfolio dashboard with a fine gapless grid of
~32px square cells whose column count scales to fill the tab width. The grid
extends past the window and the Market tab scrolls when cards are placed lower.

- Cells are a fixed square edge (>= ~32px), gapless; cards inset by 3dp so
  neighbours don't touch. Column count = availWidth / 32dp (min pfMinW cols).
- Card minimum span is pfMinW x pfMinH (~128x96); layout/resize clamps to it.
  New groups default to 6x4 cells.
- Content height reserves two extra rows below the lowest card so a card can be
  dragged past the current bottom; the scroll child grows to match.
- Dot grid (shown while rearranging) softened and extended into the buffer rows.

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

src/config/settings.h

@@ -91,12 +91,12 @@ public:
         bool show24h = false;           // show the 24h % change (live-market bases only)
         bool showSparkline = false;     // show a price-trend sparkline (live-market bases only)
         int sparklineInterval = 0;      // 0=minute 1=hour 2=day 3=week 4=month (resample of price history)
-        // Dashboard grid placement on the Market portfolio. col/row in square grid cells
-        // (-1 = auto-place), w/h in cells (span; minimum 2x1). Set when the user drags/resizes.
+        // Dashboard grid placement on the Market portfolio. col/row in fine ~32px square cells
+        // (-1 = auto-place), w/h in cells (span). Set when the user drags/resizes a card.
         int gridCol = -1;
         int gridRow = -1;
-        int gridW = 2;
-        int gridH = 1;
+        int gridW = 6;
+        int gridH = 4;
     };
 
     // Theme