fix(card_drag): extend drop rects one trailing row so beyond-extent drops land

computeSlotRectsForDrop adds cols slots past maxSlot; classifyCardDrag and
drawCardDropTarget use it instead of the render rects. doReorderDrop already
accepts any non-negative slot. New tests cover beyond-extent resolves,
empty-bank trailing row, gap pixels still miss, empty-cell within-extent works.

src/card_drag.cpp

@@ -73,6 +73,16 @@ std::vector<SlotCellRect> computeSlotRects(int maxSlot, int panelWidth,
     return rects;
 }
 
+std::vector<SlotCellRect> computeSlotRectsForDrop(int maxSlot, int panelWidth,
+                                                   const GridSpec& spec) {
+    const int cols = columnsForWidth(panelWidth, spec);
+    // One trailing row of slots past the last occupied slot — the drop-target extension.
+    // When maxSlot < 0 (empty bank) the trailing row begins at slot 0.
+    const int firstTrailing = maxSlot + 1;
+    const int newMax = firstTrailing + cols - 1;  // fills one full trailing row
+    return computeSlotRects(newMax, panelWidth, spec);
+}
+
 int hitTestSlot(int px, int py, const std::vector<SlotCellRect>& rects) {
     for (const SlotCellRect& r : rects) {
         // Half-open bounds so adjacent rects never both claim a pixel.