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

Game.tscn

@@ -190,17 +190,11 @@ autowrap_mode = 3
 layer = 5
 
 [node name="InventoryBackpack" parent="HUD" unique_id=-294967294 instance=ExtResource("9_backpack")]
-anchors_preset = 12
-anchor_top = 1.0
-anchor_bottom = 1.0
-offset_top = -64.65503
-offset_bottom = -4.654907
-grow_vertical = 0
 
 [node name="InventoryOverlayLayer" type="CanvasLayer" parent="." unique_id=-294967293]
 layer = 10
-visible = false
 
 [node name="InventoryOverlay" parent="InventoryOverlayLayer" unique_id=-294967292 instance=ExtResource("a_overlay")]
-grow_horizontal = 2
-grow_vertical = 2
+
+[connection signal="overlay_show_requested" from="HUD/InventoryBackpack" to="HUD/InventoryBackpack/.." method="_on_backpack_show_overlay"]
+[connection signal="overlay_hide_requested" from="HUD/InventoryBackpack" to="HUD/InventoryBackpack/.." method="_on_backpack_hide_overlay"]