feat(UI): 优化技能图标视觉效果并调整UI布局

- 为技能图标添加粒子效果，在冷却完成时显示
- 调整冷却进度条的着色器参数，改进视觉效果
- 修改默认游戏难度为MASTER
- 重构UI布局，将技能图标整合到能量条容器中
- 调整实体升级公式，使用平方根计算难度加成

scripts/Statemachine/SkillIcon.gd

@@ -4,6 +4,10 @@ class_name SkillIcon
 @export var weapon: Weapon = null;
 
 @onready var textureRect: TextureRect = $"%texture"
+@onready var particle: GPUParticles2D = $"%particle"
+
+var showed: bool = false
+
 func _ready():
 	material = material.duplicate()
 func _physics_process(_delta):
@@ -11,3 +15,9 @@ func _physics_process(_delta):
 		textureRect.texture = weapon.avatarTexture
 		var progress = min(weapon.cooldownTimer.timeSinceLastStart() / weapon.cooldownTimer.cooldown, UIState.player.energy / weapon.needEnergy)
 		material.set_shader_parameter("progress", progress)
+		if progress >= 1:
+			if !showed:
+				showed = true
+				particle.emitting = true
+		else:
+			showed = false