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/autoload/SaveManager.gd

@@ -25,7 +25,7 @@ func load_game() -> void:
 		return
 	var raw: String = file.get_as_text()
 	file.close()
-	var parsed = JSON.parse_string(raw)
+	var parsed: Variant = JSON.parse_string(raw)
 	if parsed is Dictionary:
 		GameState.apply_save_data(parsed)