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/characters/character.gd

@@ -25,4 +25,4 @@ func _on_drag_picked_up(_pos: Vector2) -> void:
 func _on_drag_released(pos: Vector2) -> void:
 	_is_held = false
 	GameState.set_character_position(character_id, global_position)
-	character_placed.emit(self, pos)
+	character_placed.emit(self, global_position)