feat: 实现存档界面与场景切换，优化文字渲染

- 新增 SavePanel/SaveSlot 存档选择界面，支持已有存档与新建存档
- 新增 GameState 单例与 MainGame 入口，按参数区分新游戏/读档
- 主菜单接入中文字体，返回/进入存档面板流程打通
- 开启 2D/3D MSAA、TAA 与屏幕空间 AA，消除放大文字锯齿
- 设置 1920x1080 视口与中文本地化过滤

src/UI/Panels/SaveSlot.gd

@@ -0,0 +1,38 @@
+extends PanelContainer
+
+signal slot_pressed(save_id: String)
+
+@onready var slot_index_label: Label = %SlotIndexLabel
+@onready var save_name_label: Label = %SaveNameLabel
+@onready var meta_line1: Label = %MetaLine1
+@onready var meta_line2: Label = %MetaLine2
+@onready var delete_button: Button = %DeleteButton
+@onready var arrow_button: Button = %ArrowIcon
+
+var save_id: String = ""
+var has_data: bool = false
+
+func setup(id: String, name_text: String, meta1: String, meta2: String) -> void:
+	save_id = id
+	has_data = true
+	slot_index_label.text = id.get_slice("save_", 1)  # "save_001" -> "001"
+	save_name_label.text = name_text
+	meta_line1.text = meta1
+	meta_line2.text = meta2
+	delete_button.visible = true
+	arrow_button.visible = true
+
+func setup_empty() -> void:
+	has_data = false
+	slot_index_label.text = "+"
+	save_name_label.text = "新建存档"
+	meta_line1.text = "点击创建新的存档"
+	meta_line2.text = ""
+	delete_button.visible = false
+	arrow_button.visible = false
+
+func _on_pressed() -> void:
+	slot_pressed.emit(save_id)
+
+func _on_delete_button_button_up() -> void:
+	print("删除存档: ", save_id)