fix: 防连点产生重复训练记录 + 补充遗漏的状态重置

- finishTraining 添加 _saving 防重入，避免1.5秒延迟期间连点
- _init 补充 _last30Signaled/_last10Signaled 重置，修复
  多次训练时语音提示丢失的bug

Co-Authored-By: Claude <noreply@anthropic.com>

pages/timer/timer.js

@@ -66,8 +66,11 @@ Page({
 
     // Reset reminder state
     this._halfwaySignaled = false
+    this._last30Signaled = false
+    this._last10Signaled = false
     this._lastMinuteAt = 0
     this._lastCountdown = 0
+    this._saving = false
 
     this._timer = new Timer({
       onTick: (tick) => {
@@ -188,8 +191,14 @@ Page({
   },
 
   finishTraining() {
+    // Guard against double-tap: stop button is still visible during the
+    // 1.5s navigateBack delay, so a second tap would create a duplicate.
+    if (this._saving) return
+    this._saving = true
+
     const elapsed = this._timer.stop()
     if (elapsed < 3) {
+      this._saving = false
       wx.showToast({ title: '训练时间太短', icon: 'none', duration: 1500 })
       setTimeout(() => { wx.navigateBack() }, 1500)
       return