fix: 修复交互与通知板问题

- 修复Z轴逻辑调整精灵与物品的排序
- 渲染前强制 content_container.modulate.a = 1.0，避免内容区被透明度状态卡住导致看起来空白

scenes/prefabs/items/welcome_board.tscn

@@ -3,7 +3,7 @@
 [ext_resource type="Texture2D" uid="uid://v7loa3smfkrd" path="res://assets/materials/WelcomeBoard.png" id="2_sprite"]
 
 [sub_resource type="RectangleShape2D" id="RectangleShape2D_board"]
-size = Vector2(112, 26.5)
+size = Vector2(112, 18.5)
 
 [node name="WelcomeBoard" type="StaticBody2D"]
 collision_layer = 3
@@ -13,5 +13,5 @@ scale = Vector2(0.25, 0.25)
 texture = ExtResource("2_sprite")
 
 [node name="CollisionShape2D" type="CollisionShape2D" parent="."]
-position = Vector2(0, 18.75)
+position = Vector2(0, 14.75)
 shape = SubResource("RectangleShape2D_board")