feat(command-zone): add Z-value constants for command zones

Extend z_values.h with command-zone specific constants:

ZValues namespace additions:
- TOGGLE_BUTTON (0.5): Button above zone background, below cards
- PARTNER_ZONE (1.0): Partner zone level within command zone
- COMMAND_ZONE (1.0): Standard zone level for command zones
- TAX_COUNTERS (overlay+2): Commander tax counter overlay layer

ZoneSizes namespace (new):
- COMMAND_ZONE_HEIGHT: Card height plus 8px padding
- MINIMUM_STACK_HEIGHT: 100px minimum for usability
- TAX_COUNTER_SIZE: 24px for tax counter icons
- COMMAND_ZONE_WIDTH: 100.0 pixels
- MINIMIZED_HEIGHT_RATIO: 25% when minimized

GameColors namespace (new):
- OVERLAY_ALPHA: 191 (75% opaque)
- OVERLAY_BG_NORMAL/HOVERED: Consistent overlay styling

These constants enable visual consistency across command zone components.

cockatrice/src/game/z_values.h

@@ -1,8 +1,11 @@
 #ifndef Z_VALUES_H
 #define Z_VALUES_H
 
+#include "board/abstract_card_item.h"
 #include "z_value_layer_manager.h"
 
+#include <QColor>
+
 /**
  * @file z_values.h
  * @brief Centralized Z-value constants for rendering layer order.
@@ -27,21 +30,112 @@
 namespace ZValues
 {
 
-// Expose base for callers that need it
+// ============================================================================
+// Zone-Level Layering (relative to parent containers)
+// ============================================================================
+
+/// Toggle button renders above zone background but below cards (child of partner zone)
+constexpr qreal TOGGLE_BUTTON = 0.5;
+
+/// Partner zone level within command zone
+constexpr qreal PARTNER_ZONE = 1.0;
+
+/// Command zone sits at standard zone level
+constexpr qreal COMMAND_ZONE = 1.0;
+
+// ============================================================================
+// Card Layering (cards use sequential indices starting from this base)
+// ============================================================================
+
+/// Base Z-value for cards in zones (actual Z = CARD_BASE + index)
+constexpr qreal CARD_BASE = 1.0;
+
+/// Maximum Z-value for cards on the table zone (see ZValueLayerManager for formula)
+constexpr qreal CARD_MAX = ZValueLayerManager::CARD_Z_VALUE_MAX;
+
+// ============================================================================
+// Overlay Elements (always on top of regular game elements)
+// ============================================================================
+
+/// Base z-value for overlay elements. Uses ZValueLayerManager::OVERLAY_BASE.
+/// Must exceed CARD_MAX to ensure overlays render above all cards.
 constexpr qreal OVERLAY_BASE = ZValueLayerManager::OVERLAY_BASE;
 
-// Overlay layer Z-values for items that should appear above normal cards
+/// Hovered card temporarily promotes to this layer
 constexpr qreal HOVERED_CARD = ZValueLayerManager::overlayZValue(1.0);
-constexpr qreal ARROWS = ZValueLayerManager::overlayZValue(3.0);
-constexpr qreal ZONE_VIEW_WIDGET = ZValueLayerManager::overlayZValue(4.0);
-constexpr qreal DRAG_ITEM = ZValueLayerManager::overlayZValue(5.0);
-constexpr qreal DRAG_ITEM_CHILD = ZValueLayerManager::overlayZValue(6.0);
-constexpr qreal TOP_UI = ZValueLayerManager::overlayZValue(7.0);
 
-// Card layering (general architecture, not command-zone specific)
-constexpr qreal CARD_BASE = 1.0;
-constexpr qreal CARD_MAX = ZValueLayerManager::CARD_Z_VALUE_MAX;
+/// Commander and partner tax counters overlay
+constexpr qreal TAX_COUNTERS = ZValueLayerManager::overlayZValue(2.0);
+
+/// Targeting arrows between cards/players
+constexpr qreal ARROWS = ZValueLayerManager::overlayZValue(3.0);
+
+/// Zone view widget (deck/graveyard viewer)
+constexpr qreal ZONE_VIEW_WIDGET = ZValueLayerManager::overlayZValue(4.0);
+
+/// Drag items and search UI elements
+constexpr qreal DRAG_ITEM = ZValueLayerManager::overlayZValue(5.0);
+
+/// Child drag items (stacked above parent drag)
+constexpr qreal DRAG_ITEM_CHILD = ZValueLayerManager::overlayZValue(6.0);
+
+/// Highest layer - group selectors
+constexpr qreal TOP_UI = ZValueLayerManager::overlayZValue(7.0);
 
 } // namespace ZValues
 
+// ============================================================================
+// Zone Size Constants
+// ============================================================================
+
+/**
+ * @namespace ZoneSizes
+ * @brief Centralized size constants for game zone dimensions.
+ *
+ * These constants define the dimensions of various zones to ensure
+ * consistent sizing across the codebase and prevent duplication.
+ */
+namespace ZoneSizes
+{
+
+/// Height of the command zone (accommodates a card plus padding)
+constexpr int COMMAND_ZONE_HEIGHT = CARD_HEIGHT + 8;
+
+/// Minimum height for the stack zone to ensure usability
+constexpr int MINIMUM_STACK_HEIGHT = 100;
+
+/// Size of commander tax counter icons
+constexpr int TAX_COUNTER_SIZE = 24;
+
+/// Width of the command zone
+constexpr qreal COMMAND_ZONE_WIDTH = 100.0;
+
+/// Height ratio when command zone is minimized (25%)
+constexpr qreal MINIMIZED_HEIGHT_RATIO = 0.25;
+
+} // namespace ZoneSizes
+
+// ============================================================================
+// Game Color Constants
+// ============================================================================
+
+/**
+ * @namespace GameColors
+ * @brief Centralized color constants for overlay UI elements.
+ *
+ * These constants ensure visual consistency between related UI elements
+ * like the partner zone toggle button and commander tax counters.
+ */
+namespace GameColors
+{
+
+/// Alpha value for overlay elements (75% opaque)
+constexpr int OVERLAY_ALPHA = 191;
+
+/// Background colors for overlay elements
+inline const QColor OVERLAY_BG_NORMAL{40, 40, 40, OVERLAY_ALPHA};
+inline const QColor OVERLAY_BG_HOVERED{70, 70, 70, OVERLAY_ALPHA};
+
+} // namespace GameColors
+
 #endif // Z_VALUES_H