Refactor/z value constants (#6651)

* feat(z-values): add centralized Z-value constants infrastructure

Magic numbers scattered across the codebase make Z-value layering
hard to understand and maintain. Centralizing them provides:
- Self-documenting layer hierarchy
- Validation utilities for development
- Single source of truth for Z-value ranges

Two-tier header design:
- z_value_layer_manager.h: Foundation with constants and validation
- z_values.h: User-facing namespace with semantic constants

* refactor(z-values): replace magic Z-value numbers with ZValues constants

Magic numbers like 2000000007 are impossible to understand without
context. Named constants (ZValues::DRAG_ITEM) are self-documenting.

This intentionally renumbers overlay Z-values to use a cleaner offset
sequence. The relative stacking order is preserved, which is what
matters for correct rendering.

Each consumer now includes z_values.h and uses semantic constants
instead of magic numbers.

* refactor(z-values): removed redundant inline with contexpr, Updated doc, magic numbers removed from TableZone.

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();