fix: wire backpack signals and simplify positioning

- Connect overlay_show_requested/overlay_hide_requested signals in Game.tscn
  to new handlers in MainGame.gd that call show_overlay()/hide_overlay()
- Simplify backpack to fixed 70x70 position at top-left (layout_mode=0)
- Remove broken anchor overrides from Game.tscn that were anchoring to bottom

inventory/inventory_backpack/InventoryBackpack.tscn

@@ -3,11 +3,8 @@
 [ext_resource type="Script" uid="uid://2x3g0ethsdcgo" path="res://inventory/inventory_backpack/InventoryBackpack.gd" id="1"]
 
 [node name="InventoryBackpack" type="Control" unique_id=1000000001]
-layout_mode = 3
-anchors_preset = 2
-anchor_right = 1.0
-offset_top = 10.0
-offset_right = -70.0
+layout_mode = 0
+offset_right = 70.0
 offset_bottom = 70.0
 script = ExtResource("1")