feat(UI): 添加游戏开始界面和难度选择功能

实现游戏开始界面，包含难度选择滑块和开始按钮
修改游戏规则初始难度为10并调整掉落物品数量计算
添加游戏开始条件检查，确保运行时间超过3秒且没有敌人时显示制作饲料界面

components/Scenes/UI.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=24 format=3 uid="uid://dfwg750a47ggx"]
+[gd_scene load_steps=25 format=3 uid="uid://dfwg750a47ggx"]
 
 [ext_resource type="PackedScene" uid="uid://ofpg5s3j7esv" path="res://components/UI/BossBar.tscn" id="1_2pe58"]
 [ext_resource type="Script" path="res://scripts/Statemachine/UIState.gd" id="1_f00a6"]
@@ -13,6 +13,7 @@
 [ext_resource type="PackedScene" uid="uid://cksonrrx38k36" path="res://components/Scenes/FullscreenPanels/Pause.tscn" id="12_jkm6o"]
 [ext_resource type="PackedScene" uid="uid://dekcqdhrjs07u" path="res://components/Scenes/FullscreenPanels/Weapon.tscn" id="13_bmc24"]
 [ext_resource type="PackedScene" uid="uid://w2o3klbsssq3" path="res://components/Scenes/FullscreenPanels/Thanks.tscn" id="14_gp8lx"]
+[ext_resource type="PackedScene" uid="uid://c0r1i5tdbwljj" path="res://components/Scenes/FullscreenPanels/Starter.tscn" id="14_pjwgj"]
 
 [sub_resource type="StyleBoxEmpty" id="StyleBoxEmpty_12otr"]
 
@@ -310,3 +311,8 @@ offset_bottom = 0.0
 [node name="Thanks" parent="root/panels" instance=ExtResource("14_gp8lx")]
 visible = false
 layout_mode = 1
+
+[node name="Starter" parent="root/panels" instance=ExtResource("14_pjwgj")]
+visible = false
+z_index = 2
+layout_mode = 1