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.wxss

@@ -155,24 +155,56 @@
 .confetti-piece {
   position: absolute;
   top: -20rpx;
-  width: 16rpx;
-  height: 32rpx;
   border-radius: 4rpx;
   animation: confettiFall 2.4s ease-out forwards;
 }
 
-.c1  { left: 10%;  background: #FF6B35; animation-delay: 0.00s; transform: rotate(15deg); }
-.c2  { left: 22%;  background: #FFD93D; animation-delay: 0.10s; transform: rotate(-20deg); }
-.c3  { left: 34%;  background: #4CAF50; animation-delay: 0.20s; transform: rotate(30deg); }
-.c4  { left: 46%;  background: #2979FF; animation-delay: 0.05s; transform: rotate(-10deg); }
-.c5  { left: 58%;  background: #EC407A; animation-delay: 0.15s; transform: rotate(45deg); }
-.c6  { left: 70%;  background: #7E57C2; animation-delay: 0.25s; transform: rotate(-30deg); }
-.c7  { left: 82%;  background: #FF8C5A; animation-delay: 0.08s; transform: rotate(20deg); }
-.c8  { left: 14%;  background: #5C9CFF; animation-delay: 0.18s; transform: rotate(-40deg); }
-.c9  { left: 38%;  background: #6EC072; animation-delay: 0.28s; transform: rotate(50deg); }
-.c10 { left: 90%;  background: #FFD93D; animation-delay: 0.12s; transform: rotate(-25deg); }
+/* Higher celebration levels: faster / longer fall */
+.confetti-piece--milestone,
+.confetti-piece--first,
+.confetti-piece--record {
+  animation-duration: 3.2s;
+}
+
+.confetti-piece--first {
+  animation-duration: 4s;
+}
 
 @keyframes confettiFall {
   0%   { opacity: 1; transform: translateY(0) rotate(0deg); }
   100% { opacity: 0.6; transform: translateY(100vh) rotate(720deg); }
 }
+
+/* ---- achievement banner ---- */
+.achievement-banner {
+  position: fixed;
+  top: 160rpx;
+  left: 50%;
+  transform: translateX(-50%);
+  background: linear-gradient(135deg, var(--primary), var(--primary-light));
+  color: #FFFFFF;
+  font-size: 32rpx;
+  font-weight: 700;
+  padding: 24rpx 48rpx;
+  border-radius: 48rpx;
+  box-shadow: 0 8rpx 24rpx rgba(var(--primary-rgb), 0.4);
+  z-index: 10000;
+  animation: achievementIn 0.5s cubic-bezier(0.34, 1.56, 0.64, 1) both,
+             achievementOut 0.4s ease-in 3.5s both;
+  white-space: nowrap;
+}
+
+.achievement-text {
+  display: block;
+  line-height: 1;
+}
+
+@keyframes achievementIn {
+  0%   { opacity: 0; transform: translateX(-50%) scale(0.5); }
+  100% { opacity: 1; transform: translateX(-50%) scale(1); }
+}
+
+@keyframes achievementOut {
+  0%   { opacity: 1; transform: translateX(-50%) scale(1); }
+  100% { opacity: 0; transform: translateX(-50%) scale(0.8); }
+}