fix(poc): resolve evaluator and review findings

- Add RectangleShape2D to Character and InteractiveObject collision areas
- Fix HUD button signal connections in _ready()
- Fix character_placed signal emitting global_position
- Extract DEFAULT_DRAG_RADIUS constant in DragDropComponent
- Type Variant on JSON parsed variable in SaveManager
- Extract music symbol constants in HUD
- Refactor duplicated drag input code in InputManager

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/systems/drag_drop_component.gd

@@ -6,6 +6,7 @@ signal drag_released(global_position: Vector2)
 
 const DRAG_Z_INDEX: int = 10
 const DRAG_SCALE: float = 1.1
+const DEFAULT_DRAG_RADIUS: float = 64.0
 
 @export var drag_target: Node2D
 
@@ -73,8 +74,8 @@ func _is_position_over_target(screen_pos: Vector2) -> bool:
 	var local_pos: Vector2 = drag_target.to_local(world_pos)
 	var area: Area2D = drag_target.get_node_or_null("CollisionArea") as Area2D
 	if area == null:
-		return local_pos.length() < 64.0
+		return local_pos.length() < DEFAULT_DRAG_RADIUS
 	for child in area.get_children():
 		if child is CollisionShape2D and child.shape != null:
 			return child.shape.get_rect().has_point(local_pos)
-	return local_pos.length() < 64.0
+	return local_pos.length() < DEFAULT_DRAG_RADIUS