feat(market): portfolio groups as a draggable/resizable dashboard grid

Turn the portfolio group area into a dashboard grid:
- Responsive column count (1-4 by width); each 1x1 cell is one card.
- Drag a card to move it to another cell; drag its bottom-right corner to
  resize (span multiple cells). Both snap to the grid.
- A subtle dot grid appears while rearranging (dragging or resizing), with a
  highlighted drop-target / resize footprint.
- Click (no drag past a 4px threshold) still opens the group editor.
- Grid placement (col/row) and span (w/h) persist per group in settings.json;
  positions are resolved each frame — stored placement when it fits, else
  auto-placed row-major into free cells. Dropping onto an occupied cell swaps.

PortfolioEntry gains grid_col/grid_row/grid_w/grid_h. The card render was
refactored into a drawCard() lambda reused for the in-grid, dragged, and
resizing states.

Full-node + Lite build clean; ctest 1/1; hygiene clean. This is an interactive
feature I can't verify without the GUI — needs drag/resize testing.

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

src/config/settings.h

@@ -90,6 +90,12 @@ public:
         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)
+        // Dashboard grid placement on the Market portfolio. col/row in grid 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 = 1;
+        int gridH = 1;
     };
 
     // Theme