fix: 振动定时器清理、完成保存守卫、暗黑冷启动、头像云存储

- timer: 振动 setTimeout 入栈追踪,暂停/停止/卸载/重练统一清理,
  避免页面销毁后残留震动(_clearVibrateTimers)
- timer: _saving 守卫统一到 _saveAndShowCompletion,自然完成与手动
  结束两路径都防重复保存;修复 onTrainAgain 不重置会话状态导致
  再练一次后第二次无语音/振动且保存被挡
- app.wxss: @media dark 补全 --text/--card-bg/--bg-soft/--border/
  --success/--danger,冷启动即为完整深色,不再背景深卡片白的割裂闪烁
- settings: 头像优先上传云存储存 fileID,三级回退(fileID->base64->
  临时URL),pushAll 不再每次传输图片字节

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

app.wxss

@@ -16,12 +16,24 @@ page {
   transition: background-color 0.3s ease, color 0.3s ease;
 }
 
-/* dark mode overrides (only for system dark mode; user choice goes through themeStyle) */
+/* dark mode overrides (only for system dark mode; user choice goes through
+   themeStyle at runtime). Kept in sync with the dark palette in
+   utils/theme.js getThemeStyle(isDark=true) so cold-start (before JS runs)
+   already shows a complete dark palette, not just the background. */
 @media (prefers-color-scheme: dark) {
   page {
+    --text: #E5E5E7;
+    --text-secondary: #98989F;
     --bg: #0F0F12;
     --bg-gradient-start: #0F0F12;
     --bg-gradient-end: #14141A;
+    --card-bg: #1C1C1E;
+    --bg-soft: #2C2C2E;
+    --border: #2C2C2E;
+    --success: #30D158;
+    --success-rgb: 48,209,88;
+    --danger: #FF6B6B;
+    --danger-rgb: 255,107,107;
   }
 }