feat(voice): 语音参数移入 config.js + 设置页男女声切换

- config: 新增 tts.female/male 两组参数(voiceType/volume/speed),
  直接改数字即可调音色,无需改逻辑代码
- storage: getSettings 兜底 voiceGender(旧用户迁移为女声)
- voice: 新增 _getTtsParams 读 config+性别;缓存 key 含完整参数,
  改音色/音量/性别自动重新合成,无需手动清缓存
- settings: 训练偏好加"语音音色"segmented(女声/男声)
- tts 云函数: _synthesize 用传入 opts 合成;_upload 路径含 voiceType
  (男/女不同文件);需重新部署

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

utils/storage.js

@@ -164,10 +164,20 @@ const getTotalStats = () => {
   return { totalDuration, totalSessions, maxDuration }
 }
 
-const getSettings = () => wx.getStorageSync(SETTINGS_KEY) || {
-  planId: 'beginner',
-  voiceGuide: true,
-  vibrate: true
+const getSettings = () => {
+  const s = wx.getStorageSync(SETTINGS_KEY)
+  if (s) {
+    // Backfill voiceGender for existing users (added with the male/female
+    // voice option). Defaults to female, the prior behavior.
+    if (!s.voiceGender) s.voiceGender = 'female'
+    return s
+  }
+  return {
+    planId: 'beginner',
+    voiceGuide: true,
+    vibrate: true,
+    voiceGender: 'female'
+  }
 }
 
 const saveSettings = (settings) => {