fix: bind callbacks before passing to tween_callback, tighten backpack hitbox

- Godot 4's CallbackTweener doesn't have .bind() — move .bind() call
  inside tween_callback() for 3 callback sites
- Shrink InventoryBackpack Control to 60x60 (offset_right -70) so click
  area matches the visible BackpackIcon square

inventory/inventory_backpack/InventoryBackpack.tscn

@@ -7,7 +7,7 @@ layout_mode = 3
 anchors_preset = 2
 anchor_right = 1.0
 offset_top = 10.0
-offset_right = -10.0
+offset_right = -70.0
 offset_bottom = 70.0
 script = ExtResource("1")