fix: 修复 hand_area 节点路径错误

- HandArea 现在在 ScrollContainer 下，路径改为 $ScrollContainer/HandArea
- 添加按钮空值检查，防止连接信号时出错

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src/ui/scenes/training_room.gd

@@ -2,7 +2,7 @@ extends Control
 
 var controller: TrainingController
 
-@onready var hand_area := $HandArea as HandArea
+@onready var hand_area := $ScrollContainer/HandArea as HandArea
 @onready var play_button: Button = $Buttons/PlayButton
 @onready var pass_button: Button = $Buttons/PassButton
 @onready var hint_button: Button = $Buttons/HintButton
@@ -10,8 +10,11 @@ var controller: TrainingController
 @onready var guide_label: Label = $GuideLabel
 
 func _ready() -> void:
+	if play_button:
 		play_button.pressed.connect(_on_play_pressed)
+	if pass_button:
 		pass_button.pressed.connect(_on_pass_pressed)
+	if hint_button:
 		hint_button.pressed.connect(_on_hint_pressed)
 	start_training()