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.cpp

@@ -332,6 +332,14 @@ bool Settings::load(const std::string& path)
                     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 (e.contains("grid_col") && e["grid_col"].is_number_integer())
+                    entry.gridCol = e["grid_col"].get<int>();
+                if (e.contains("grid_row") && e["grid_row"].is_number_integer())
+                    entry.gridRow = e["grid_row"].get<int>();
+                if (e.contains("grid_w") && e["grid_w"].is_number_integer())
+                    entry.gridW = e["grid_w"].get<int>();
+                if (e.contains("grid_h") && e["grid_h"].is_number_integer())
+                    entry.gridH = e["grid_h"].get<int>();
                 if (!entry.label.empty()) portfolio_entries_.push_back(std::move(entry));
             }
         }
@@ -490,6 +498,10 @@ bool Settings::save(const std::string& path)
         entry["show_value"] = e.showValue;
         entry["show_24h"] = e.show24h;
         entry["show_sparkline"] = e.showSparkline;
+        entry["grid_col"] = e.gridCol;
+        entry["grid_row"] = e.gridRow;
+        entry["grid_w"] = e.gridW;
+        entry["grid_h"] = e.gridH;
         j["portfolio_entries"].push_back(std::move(entry));
     }
     j["font_scale"]     = font_scale_;