feat: 完成训练分级庆祝 (normal/first/milestone/record)

- normal: 12 条彩带
- record (破个人纪录): 24 条 + 横幅"🏆 新纪录!X秒"
- milestone (连击 7/14/30/50/100/365 天): 32 条 + 横幅
- first (首次训练): 40 条 + 横幅"🎉 第一次训练"

横幅 3.5s 后自动淡出,彩带时长随等级递增(2.4s → 4s)。

pages/timer/timer.wxml

@@ -2,16 +2,17 @@
 
   <!-- 全屏彩带完成庆祝 -->
   <view class="confetti" wx:if="{{isCompleted}}">
-    <view class="confetti-piece c1"></view>
-    <view class="confetti-piece c2"></view>
-    <view class="confetti-piece c3"></view>
-    <view class="confetti-piece c4"></view>
-    <view class="confetti-piece c5"></view>
-    <view class="confetti-piece c6"></view>
-    <view class="confetti-piece c7"></view>
-    <view class="confetti-piece c8"></view>
-    <view class="confetti-piece c9"></view>
-    <view class="confetti-piece c10"></view>
+    <view
+      wx:for="{{confettiPieces}}"
+      wx:key="i"
+      class="confetti-piece confetti-piece--{{celebrationLevel}}"
+      style="left: {{item.left}}%; background: {{item.color}}; animation-delay: {{item.delay}}s; transform: rotate({{item.rotate}}deg); width: {{item.size}}rpx; height: {{item.size * 2}}rpx;"
+    ></view>
+  </view>
+
+  <!-- 成就横幅 (record / milestone / first) -->
+  <view class="achievement-banner" wx:if="{{isCompleted && celebrationLevel !== 'normal'}}">
+    <text class="achievement-text">{{celebrationMessage}}</text>
   </view>
 
   <!-- 呼吸引导环 + 进度条 -->