fix(vibrate): 修复旧用户震动失效 + 新增震动强度选项

根因:user_settings 早期无 vibrate 字段,getSettings 未兜底,
_remind 的 !s.vibrate 误判 undefined 为关闭,导致震动静默失效,
需手动操作开关才恢复。

- storage: getSettings 兜底 vibrate/voiceGuide/vibrateIntensity 默认值
- timer: !s.vibrate 改 === false,字段缺失视为开(与 voiceGuide 一致)
- timer: vibrate 按 vibrateIntensity 选 API(light/medium=vibrateShort,
  heavy=vibrateLong),fail 回调辅助排查
- settings: 新增"震动强度"三档(短促/标准/强烈),默认强烈

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

utils/storage.js

@@ -170,13 +170,21 @@ const getSettings = () => {
     // Backfill voiceGender for existing users (added with the male/female
     // voice option). Defaults to female, the prior behavior.
     if (!s.voiceGender) s.voiceGender = 'female'
+    // Backfill vibrate/voiceGuide for users whose settings predate these
+    // toggles. Without this, s.vibrate is undefined and `!s.vibrate` in
+    // the timer reads it as "off" - vibration silently no-ops until the
+    // user manually toggles the switch (which writes a real boolean).
+    if (s.vibrate == null) s.vibrate = true
+    if (s.voiceGuide == null) s.voiceGuide = true
+    if (!s.vibrateIntensity) s.vibrateIntensity = 'heavy'
     return s
   }
   return {
     planId: 'beginner',
     voiceGuide: true,
     vibrate: true,
-    voiceGender: 'female'
+    voiceGender: 'female',
+    vibrateIntensity: 'heavy'
   }
 }