refactor(graphics): replace Z-value magic numbers with constants

Replace hardcoded Z-value numbers with ZValues::* constants:
- 2000000007 -> ZValues::DRAG_ITEM
- 2000000007 + offset -> ZValues::DRAG_ITEM_CHILD
- 2000000004 -> ZValues::HOVERED_CARD
- 2000000005 -> ZValues::ARROWS

This improves code readability by making the intent of these layer
values explicit and centralizes Z-ordering logic for easier maintenance.

cockatrice/src/game/board/abstract_card_drag_item.cpp

@@ -1,6 +1,7 @@
 #include "abstract_card_drag_item.h"
 
 #include "../../client/settings/cache_settings.h"
+#include "../z_values.h"
 
 #include <QCursor>
 #include <QDebug>
@@ -18,13 +19,13 @@ AbstractCardDragItem::AbstractCardDragItem(AbstractCardItem *_item,
 {
     if (parentDrag) {
         parentDrag->addChildDrag(this);
-        setZValue(2000000007 + hotSpot.x() * 1000000 + hotSpot.y() * 1000 + 1000);
+        setZValue(ZValues::DRAG_ITEM_CHILD);
         connect(parentDrag, &QObject::destroyed, this, &AbstractCardDragItem::deleteLater);
     } else {
         hotSpot = QPointF{qBound(0.0, hotSpot.x(), static_cast<qreal>(CARD_WIDTH - 1)),
                           qBound(0.0, hotSpot.y(), static_cast<qreal>(CARD_HEIGHT - 1))};
         setCursor(Qt::ClosedHandCursor);
-        setZValue(2000000007);
+        setZValue(ZValues::DRAG_ITEM);
     }
     if (item->getTapped())
         setTransform(QTransform()

cockatrice/src/game/board/abstract_card_item.cpp

@@ -3,6 +3,7 @@
 #include "../../client/settings/cache_settings.h"
 #include "../../interface/card_picture_loader/card_picture_loader.h"
 #include "../game_scene.h"
+#include "../z_values.h"
 
 #include <QCursor>
 #include <QGraphicsScene>
@@ -215,7 +216,7 @@ void AbstractCardItem::setHovered(bool _hovered)
     if (_hovered)
         processHoverEvent();
     isHovered = _hovered;
-    setZValue(_hovered ? 2000000004 : realZValue);
+    setZValue(_hovered ? ZValues::HOVERED_CARD : realZValue);
     setScale(_hovered && SettingsCache::instance().getScaleCards() ? 1.1 : 1);
     setTransformOriginPoint(_hovered ? CARD_WIDTH / 2 : 0, _hovered ? CARD_HEIGHT / 2 : 0);
     update();

cockatrice/src/game/board/arrow_item.cpp

@@ -5,6 +5,7 @@
 #include "../player/player.h"
 #include "../player/player_actions.h"
 #include "../player/player_target.h"
+#include "../z_values.h"
 #include "../zones/card_zone.h"
 #include "card_item.h"
 
@@ -23,7 +24,7 @@ ArrowItem::ArrowItem(Player *_player, int _id, ArrowTarget *_startItem, ArrowTar
     : QGraphicsItem(), player(_player), id(_id), startItem(_startItem), targetItem(_targetItem), targetLocked(false),
       color(_color), fullColor(true)
 {
-    setZValue(2000000005);
+    setZValue(ZValues::ARROWS);
 
     if (startItem)
         startItem->addArrowFrom(this);