feat: implement InventoryBackpack FSM and InventoryOverlay

- InventoryBackpack: Control-based FSM with IDLE/OPEN/SELECTED/ACQUIRE/REMOVE
  states, Tween-based animations, guard condition checks, signal connections
  to InventoryManager for item_acquired/item_removed reactions
- InventoryOverlay: Full-screen overlay with fade-in/out, item grid via
  GridContainer, drag-and-drop item selection, combination via drag-to-slot,
  hover labels, right-click inspect
- InventorySlot: Individual slot with colored box placeholder, hover highlight,
  click/right-click/hover signals

inventory/inventory_overlay/InventoryOverlay.tscn

@@ -0,0 +1,63 @@
+[gd_scene format=3 uid="uid://1p46uzngsih9o"]
+
+[ext_resource type="Script" uid="uid://3mkdj9s1oe1jz" path="res://inventory/inventory_overlay/InventoryOverlay.gd" id="1"]
+
+[sub_resource type="LabelSettings" id="LabelSettings_inv"]
+font_size = 20
+outline_size = 3
+outline_color = Color(0, 0, 0, 1)
+
+[node name="InventoryOverlay" type="Control" unique_id=3000000001]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+script = ExtResource("1")
+
+[node name="Background" type="ColorRect" parent="." unique_id=3000000002]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+color = Color(0, 0, 0, 0.7)
+
+[node name="InventoryPanel" type="Control" parent="." unique_id=3000000003]
+layout_mode = 1
+anchors_preset = 7
+anchor_left = 0.5
+anchor_top = 0.5
+anchor_right = 0.5
+anchor_bottom = 0.5
+offset_left = -350.0
+offset_top = -150.0
+offset_right = 350.0
+offset_bottom = 150.0
+grow_horizontal = 2
+grow_vertical = 2
+
+[node name="Frame" type="ColorRect" parent="InventoryPanel" unique_id=3000000004]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+color = Color(0.15, 0.15, 0.2, 1)
+
+[node name="ItemGrid" type="GridContainer" parent="InventoryPanel" unique_id=3000000005]
+layout_mode = 1
+anchors_preset = 0
+offset_left = 10.0
+offset_top = 10.0
+offset_right = -10.0
+offset_bottom = -50.0
+
+[node name="HoverLabel" type="Label" parent="InventoryPanel" unique_id=3000000006]
+layout_mode = 1
+anchors_preset = 8
+anchor_left = 0.0
+anchor_top = 1.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+offset_bottom = -10.0
+grow_horizontal = 2
+horizontal_alignment = 1
+label_settings = SubResource("LabelSettings_inv")