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

MainGame.gd

@@ -74,6 +74,12 @@ func restore_cursor() -> void:
 func _unhandled_input(event: InputEvent) -> void:
 	$SceneViewport.push_input(event)
 	
+func _on_backpack_show_overlay() -> void:
+	$HUD/InventoryOverlay.show_overlay()
+
+func _on_backpack_hide_overlay() -> void:
+	$HUD/InventoryOverlay.hide_overlay()
+
 func _input(event):
 	if event.is_action_released("quit"):
 		get_tree().quit()