- 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
- 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
