fix: timer 后台回到前台时立即刷新 UI

WeChat 在后台冻结 setInterval,用户从训练页切走再回来时
看到的还是离开前的画面。注册 wx.onAppShow 监听器,回到
前台立即强制 _tick() 一次,UI 跳到真实 elapsed,半程/30s/10s
等延迟到达的提醒也能补触发。

utils/timer.js

@@ -12,6 +12,17 @@ class Timer {
     this._running = false
     this._paused = false
     this._completed = false
+    this._appShowBound = false
+
+    // WeChat freezes setInterval while the mini-program is in background.
+    // When the user returns, force an immediate tick so the UI jumps to
+    // the actual elapsed time (instead of staying on the last pre-background
+    // frame) and any missed halfway / 30s / 10s reminders can still fire.
+    this._onAppShow = () => {
+      if (this._running && !this._paused) {
+        this._tick()
+      }
+    }
   }
 
   start(duration) {
@@ -24,6 +35,15 @@ class Timer {
     this._startTime = Date.now()
     this._tick()
     this._intervalId = setInterval(() => { this._tick() }, 1000)
+
+    // Register foreground listener (idempotent — we keep a flag so we don't
+    // double-register if start() is called multiple times in one session)
+    if (!this._appShowBound && typeof wx.onAppShow === 'function') {
+      try {
+        wx.onAppShow(this._onAppShow)
+        this._appShowBound = true
+      } catch (e) {}
+    }
   }
 
   pause() {